Well, that's the All Hallows thing out of the way. And Guy Fawkes, too. And yes, there's been a schedule slip. I've had a busy week. Anyway, because I haven't really prepared anything, I'm going to improvise and explore Allies possibilities for the Legion, which includes using it as an ally and using allies on it.
Chaos Daemons: These are our only Battle Brother allies because we're on the wrong side of the tracks here. Key features include the Daemon Prince Mk.2, which has a cheaper base cost, although not as much once you factor wings in (75 unwinged, 55 winged) and a slightly different breath method. As you know, I'm fond of template weapons on winged Princes, which means I could use either:
Chaos Space Marines Daemon Prince: Wings, Power Armour, Daemon of Nurgle/Tzeentch, Burning Brand - 245
OR
Chaos Daemons Daemon Prince: Wings, Iron Hide, Mark of Nurgle/Tzeentch, Breath O' Chaos, Unholy Might - 245-50
Both of them work out at about the same, however one also uses up my HQ. One has Torrent on their template, but at the expense of it being weaker than Breath. One is a better ground fighter than the other. There's also the Heldrake to factor in, which with a Baleflamer essentially does the same job as both of these but for cheaper. It's also faster, but with a horrible turning circle. Therefore, the best option is neither, but I prefer the Daemons version.
I could remove things like 3+ and Might to lower costs, but then it's not safe to be grounded and the shots that do hit me hurt more. Hmm...
Daemons also have going for them cheapish HQ (although I only own a fairly useless Khorne Herald at present), but their Troops don't really add too much. Especially when you consider a Daemonette is one point higher up than basic Bolter Marines. The Tzeentch units seem like a better deal, especially since the Flamer update, but they're not high on my good list considering the models I own.
Dark Eldar: Amusingly the only Desperate Allies (how?). Because Chaos Troops are that good, all I really need is one unit of Warriors, a Raider and a Haemo and I'm started. From there I can add a Heavy Support for either an extra plane or more Lances, and some Cannon Trueborn in another Raider if I really wanted. I'm not considering Fast as I don't think it's as good as Chaos' lot.
Unfortunately, DE can't benefit from Chaos' glorious Troops, but since the tax isn't too high, I'd consider a Sorceror, Cultists, then something shiny and good since DE are fairly points-efficient themselves. I could also take a Prince if I really wanted...
Imperial Guard: Imperial Guard go with everything, and provide what are essentially sort-of tougher Cultists. Unfortunately, unlike Loyalists, you can't improve their leadership with your own HQ, nor can you buff them with your own Biomancy, relying on a Primaris or nothing.
By the way, are we to assume that Master of Deception works on all Infantry in your army, including Allies? If yes, there's fun to be had with Huron and Guardsmablobs. If no, ah well. Guardsmen suit this better than Cultists due to real Sergeant options and such.
And obviously Guard give you planes and tanks by the bucketload even with one option, so you're not struggling for choice.
Necrons: Hoo boy. Dragons and croissants. I'm at a loss where to start, really. There's almost too many options, most of them good. Generally, the Necrons are better at eliminating vehicles due to Haywire and Gauss, while Chaos can do combat protection, psychics, and anti-infantry.
Oh, and don't get a Chaos character and a Necron character with mindshackles in the same combat. You won't get the mileage out of the Lord. And due to his low I, Glorious Intervention is unlikely. Sigh... I really want to play around with this, since the two of them combined make a mega-Codex with great performance in 6th.
Orks: Ork Troops are essentially better Nurgle Cultists. They are also essentially better Khorne Cultists due to their higher base Attacks. And they don't need to pay for their basic rifley thingy. Oh, and they have yet another winged monstrosity. This is another combo I like. They even have the unconvential airbane unit that is Lootas, who cost significantly less than Flakkvocs.
Tau: Have the Hammerhead, Battlesuits aplenty and other things. But nothing really stands out for me. I need to test. Probably benefit from Chaos Allies more than Chaos benefit from Tau Allies.
I need to test, but I'm in a recovery phase right now. On Wednesday, I'll see what I can do about procuring a drill...