Probably just a month before the release of the new Codex: Chaos Space Marines, I've decided to sit down, reflect on some of the rumors and jot down a few ideas in order to inspire myself (and perhaps even you!) with the exciting, yet challenging adventure that is starting an army from the ground!  
We all know that Codex: Chaos Space Marines is already finished and it's
 just a matter of time until it is released. The rumor mill is already 
spinnin' with delicious tidbits and so far things are lookin' pretty 
promising for the arch-enemies of the Loyalists.
And while I can all but speculate about the things in the Chaos Space 
Marine codex I can at least jot down some of the more promising and 
thematic units from Codex: Imperial Guard, which will be playing the 
role of Special Operatives and various agents, indoctrinated and placed 
by the Alpha Legion..
The
 Alpha Legion epitomizes trickery, infiltration, outmaneuvering and 
outsmarting. While they are just as good (if not better) at prolonged, 
direct combat, the Alpha Legion prefers to destroy from within, to 
ambush and slowly chip away at the enemy and to manipulate using 
subterfuge and propaganda. Information is crucial and the spy networks 
organized by them are deep and complex. Basically, if each Legion was 
represented by a god from the Norse Mythology, the Alpha Legion would be
 Loki himself.
CODEX: Chaos Space Marines
What's going to be in the new (rumored to be hard-cover) codex, what's going to stay the same and what new toys we'll be getting? Your guess is as good as mine, although there are already a bunch of juicy rumors dripping like melted cheese on a medium-rare steak. Cultists, new marks, new Land Raiders, the Dragon, Daemon Engines, new tables, new rules... oh my! 
Staying Undivided
I think this is the most important part of the list-building process. The Alpha Legion is a legion undivided. They do worship Chaos, but they are not dedicated to a single god and I would even say that they don't bother with summoning daemons and other vile creatures of the Warp, because they are unpredictable and unreliable when it comes to precision strikes and years of perfectly-executed tactics. As such I will be doing my best to stick to the following limitations: 
- Only Undivided models
- Very limited Daemonic presence 
There are obviously a few things to think about when it comes to the composition of this list. It needs to be a very well-rounded list, because it is how the Alpha Legion works. This means plenty of shooting, enough assault, infiltration and of course - the presence of psykers and psychic protection. The strength of such a list would come from its multiple layers of hybridization. The opponent will have to pick from going after vehicles or infantry, and then this decision will be split between going after the Chaos Space Marine core of the list or the more vulnerable (but potent!) I.G stuff. 
I will do my best to have as much 3+ bodies on the table as I have Guardsmen. I can actually see the Alpha Legion using Razorbacks if they have access to it and even some Rhino Rushing. We shall see. 
- Keep it balanced! 
And now.. for the I.G part: 
 HQ
Company Command squad:
 The Company Command Squad is definitely a solid choice. It is 
well-rounded and offers a LOT of support to the main Alpha Legion. How 
you equip the squad and what juicy upgrades you grant them is actually 
not that important. Keep 'em cheap, for all you need are.. 
+
Astropath's 
Telepathic Relay
 You add +1 to your Reserve rolls and the ability to re-roll outflank 
sides is also amazing, because you'll be doing a lot of that. A 2+ 
rather than 3+ for your units to arrive on Turn 2 is nice, although not 
as important, but it means that all of your stuff will arrive 
automatically on Turn 3.
+
Officer of the Fleet's 
Intercept Reserves,
 makes it harder for your opponent's units to come from reserve, needing
 a
 4+ on Turn 2, and a 3+ on Turn 3, just like the good old days of 5th 
Edition. Unfortunately things arrive automatically on turn 4, despite 
any negative modifiers. I think....? The ability to re-roll outflank 
sides is also amazing, because you'll be doing a lot of that.
The
 Astropath and OotF are important, especially with how good flyers are 
in 6th Ed. Remember that all Flyers HAVE to come from reserve, so any 
chance of delaying them is worth its points. There are also other ways 
to screw with your opponent's reinforcements by purchasing a:

- 
Land Raider Proteus (
experimental rules here),
 whose Explorator Augury Web adds another -1 to the opponent's reserve 
rolls (so Turn 2 = 5+/Turn 3 = 4+) AND you can choose to re-roll your 
Reserve rolls, even if they are successful, AND it has Scout AND it's an
 Elites choice for Codex: Chaos Space Marines. Sure, it doesn't pack the
 punch of some other Land Raiders, it is not an assault vehicle and can 
carry only 8 dudes.. but it can get Ceramite Armour (immune to Melta 
special rule)

There's also a rumor that in the new Codex, Undivided Chaos Lords get Inspiring Presence (
Inspiring Presence - friendly units  within 12" can use the warlords LD) command trait, and improved Reserve manipulation (from 
Faeit 212).
 What "improved Reserve maniupation" means is a mystery, but it could 
easily be another +1/-1 to your/opponent's reserve rolls and possibly 
the ability to re-roll your own Reserves. Depending on what THAT is, one
 of the Regimental Advisers might be obsolete, freeing you 30 points. 
Such a warlord would be a MUST for an Alpha Legion-themed army.
Keep in mind, though, that I'm not 100% sure that the +1/-1 modifiers stack. The only precedent is taken from the 
I.G FAQ: 
Q. If you take two Astropaths or two Oflicers of the Fleet, do their +1/-1 to reserve rolls stack? (p31). A. No.
I
 still think that with the current way I have it, the modifiers would 
stack, since they come from different abilities with different names, 
from different units. Not stacking would be lamesauce. 
+Ursarkar Creed's Tactical Genius
 is also pretty obvious, though including him in your army will tip the 
scales towards the I.G detachment and you need 2,000 pt., if you want 
him and the Regimental Advisers from the Company Command squad. He's a 
great leader, but you also would need a bunch of I.G squads to benefit 
from his order shenanigans. Keep in mind that Tactical Genius allows you
 to give the Scout USR to an infantry or vehicle unit in your army. So 
you can even give Scouts to a big-ass unit of Chaos Space Marines.. or 
whatever tickles your fancy. Heh. 
Do not bother with Krell. He's not worth it in this list. 
+Last but not least is the Primaris Psyker.
 The Primaris Psyker is awesome, because his ability to grab Telepathy 
and swap both of his meh powers, since Lightning Arc sucks and 
Nightshroud is situational and too random. Using the Primaris Psyker 
pretty much assumes that he's attached to a Psyker Battle Squad. You 
take the Primaris Power (Psychic Shreak - 12'' witchfire. Roll 3d6, 
subtract target's Ld and you deal that many wounds, no armor or cover 
allowed) and roll for a random power.. because they are ALL good and 
WILL benefit you greatly. You really want Terrify, though.. which combos
 greatly with the Psyker Battle Squad's Weaken Resolve. No unit is safe 
from your psychic shenanigans then. 
While
 this unit might be incredibly cheesy (now even more than ever..) it 
actually makes sense in the Alpha Legion army. The Alpha Legion is 
famous for using psykers for intelligence, spying and all sorts of vile 
things.
ELITES
+Guardsman Marbo: A MUST-HAVE!!!!! Seriously.
 Marbo epitomizes the whole infiltration, sleeper cell agent-doctrine. 
He's the sneakiest, the trickiest of them all and has all the powers 
cherished by the Alpha Legion. He's Fearless, Stealth, Move Through 
Cover, Hit and Run, has Fleet and basically can Deep Strike wherever he 
wants to, without scattering. What makes him even better, though is his 
friggin Demolition Charge, which.. by the way have gotten waaay better! 
Assault 1, One Shot, 6'' range S8, AP2, LARGE BLAST, on a model with 
BS5. That's pretty amazing. Use Marbo as a one-trick pony. Get him in 
cover and within an inch of a delicious target (Preferably Paladins - 
ignores their armor, FnP and causes Instant Death) and drop the 
template. Hope for the best. On average, 2/3's of the time it will 
scatter 2''. So hopefully plan for that.  IF Marbo survives the next 
turn.. well.. consider yourself lucky and just do what Alpha Legion does
 best.. wreak havoc.  
+Psyker Battle Squad:
 Basically what I wrote for the Primaris Psyker. They work well-enough 
alone, but you really want that Marbo, so I would recommend using them 
in 2,000 pt. games, accompanied by the PP dude.  
+Storm Trooper Squad:
 They are seriously fluffy and thematic... but I'll probably never end 
up using them, simply because Marbo and the Psyker Battle Squad are 
better.. 
TROOPS 
This
 is where things become hard. I am not sure how many Troops I want, what
 weapons to give them, etc. etc. Though I imagine a Platoon Command 
Squad lead by.. 
+Al'Rahem:
 First things, first: Al'Rahem allows you to have a bunch of squads 
belonging to his Platoon to outflank. They MUST Outflank, which means 
that they don't count towards your Reserve limitations. That's nice. You
 can easily get carried away and invest a lot of points in Al'Rahem's 
platoon, but with 6th Ed and all of your Reserves shenanigans you'll be 
able to have a much more reliable outflanking unit, potentially arriving
 automatically on Turn 2 and re-rolling Outflanking sides! Al'Rahem's Like the Wind!
 order is pretty cool, actually - lets you shoot and then run. Nice for 
Rapid Firing some lasguns at a unit and then retreating, making assaults
 less possible. Also..  his weapon is nasty. Power weapon that causes 
Instant Death? Delicious. Yeah, he's a wimp in Close Combat, which is 
why you make sure to stick a Commissar with Al'Rahem to accept 
challenges. Then try to Precision Strike important models and take them 
out of commission. Forever. 
As
 I said, I have no idea how I want to run my Platoon Command Squad. Do I
 want smaller, elite units or a bigger blob? It all depends on what the 
Cultists bring to the table, I guess. Though I'm really tempted to get a
 Specialist Squad and give them Demolition Charges.. because.. bombs are cool.   
I dunno. Maybe some Flamers? 
What about Weapon Platforms... ?
+Veteran Squads - The
 weapon specialist.. Both the Forward Sentries (with Snipers and either 
Heavy Bolter/Autocannon platform) and Demoltions doctrines are cool.. 
and so is..
+Gunnery Sergeant Harker
While
 this list pretty much concludes the units that I want to include as my 
Allies detachment, there's also one more unit that I really, really want
 to have: 
HEAVY SUPPORT
+A Squadron of 3 Hydras! : Come
 on. Why the hell not? Not only is the Hydra the emblem of the Alpha 
Legion, but Hydras are simply amazing, you guys! With glances no longer 
messing up with the shooting capability of the vehicle, Hydras are crazy
 sweet. AND they have Skyfire and ignore Jink cover saves. So that's 12 twin-linked S7, AP4 Hydra autocannon shots and then 9 S5 AP4 heavy bolter shots. Great
 againt flyers which they always hit on a 4+, ignoring Jink saves; great
 against light vehicles and other squadrons which they will glance to 
death; enough shots to make infantry cry.
Some of my friends actually complained when I shared this idea with them, becoming all hurrr-durr-like by saying "This doesn't make sense, the Alpha Legion is all about sneaking and stuff and how would you have gotten Hydras? They are super advanced and stuff!!!?" 
Nonsense, I say. They are just jealous because it's fluffy, funny and actually makes sense. The Alpha Legion infiltrates other Adeptus Astartes, trifles with Daemons.. I'm sure they can get their hands on a few Hydras. Some people have no imagination these days..