I was just thinking about this, what I believe is missing from 40K is a faction like Mercenaries or Rogue Traders or something to that effect.
See, I originally wanted a Space Marines/Grey Knights/Sororitas army, because they have very cool models. An entire army of Space Marines is just the same 3 models over and over: marine, scout, terminator. Grey Knights, or at least Grey Knight Terminators, are amazing models - easily the best 40K armoured infantry in my opinion, and the SoB also have some very nice models (plus both factions allow you to take almost any models for inquisitors and henchmen), but I wouldn't want to buy, paint, or field an entire army of only one of them. So the old 'allies' rule seemed quite cool, but now it's gone and with it the chance of variety.
Yes, you can still take inquisitors and henchmen in a Grey Knight army, so I guess that's the way to go if all you want is a variety of different models. But I'm thinking of something more. An army where you can mix humans of all sizes with xenos of all shapes and colours. Something with a touch of the elitism of Space Marines (without the power armor), the range of styles possible to Imperial Guard (without being so weak and numerous or meched-up), and the visual interest and appeal of various aliens.
I'm thinking humans with more character and ability than IG. Perhaps "elite" troops would have WS4 BS4 S3 T3 W1 I4 A2 Ld8, that sort of thing, with lots of wargear and rules that cause re-rolls, to make up for their low strength and toughness and armour. Normal troops can have regular Guard stats, but a specific purpose like heavy weapons. Of course the Elites would include psykers, heavily armoured or enhanced warriors, or specialists with special wargear like snipers.
That's just the regular humans. It can also include mutants, rogue marines (defectors from either side), Ork mercenaries like the Flash Gits, Tau rogue Tau, Dark Eldar pirates, enigmatic Eldar Warlocks working towards a purpose only they understand, or Tau defectors, reprogrammed Necrons, perhaps even pet Tyranids (I'm thinking their minds have been dominated by psykers or their brains wired or replaced, like servitors).
There's a huge range of possible vehicles, with Imperial, Tau and Eldar all having some great stuff. It will probably go the ork route of having a general description of the vehicle, with a number of analogues actually being possible in modelling terms (eg a "hovering transport" will have certain rules and stats, the player can then chose between, say, a Tau Hammerhead, an Eldar Wave Serpent, or a Marine Land-Speeder model). Actually, this same idea could be applied to infantry units as well, so a unit of scouts could use Marine scouts or Pathfinder models, and each has slightly different rules or wargear (the same as sniper scouts and combat scouts play differently and are visually differentiated by their wargear). This would create variety without needing an obscene range or units.
I guess the real reason behind this idea is to create an army that is fun for people who like painting or modelling individual models, rather than large numbers of the same model over and over. You can get nicely matched models in a unit, but there's no need to ever paint more than one unit of the same model. In fact, to enforce this (and to provide a counterbalance to the large variety of units available), the codex may stipulate that you cannot have two of the same unit (or possibly you can but they need to be different races, like the Scouts and Pathfinders example above). I know that there's a fair number of people who would like an army with no spam. Or just enforce a different maximum for each unit, the way some codices do (or did? I'm thinking Inquisitor Lords in the previous Grey Knights).
Anyway, I might actually try to come up with a few rules for this army. I think I'd put most units as troops, to allow for said variety without too many unnecessary rules - instead the limits on individual units will prevent spam. It might even be worth getting rid of the army composition chart altogether. The problem is I'm really only familiar with the Space Marines and Orks codices (and some other power-armoured variants). I'll start with those anyway, and pick units or create my own based on them. Of course I'm open to suggestions.