To hell with what GW gives you in terms of fluff. Make up a race. Figure out what types of stats would bes suit your army. Then choose that race, and Counts As the whole damn thing.
My primary army are Thousand Sons, but I play them in that moral middleground such as Pre-Heresy; they never wanted Chaos, but the Imperium abandoned them for the flaw that the Emperor bred into them. Lots of robed sorcerer marines, etc etc. But still an Astartes race.
However, after that I just mix and match. I decided for my next army, I wanted Adeptus Mechanicus. So I said to myself, how would that work? I wanted to focus on super advanced tech and fairly advanced cybernetics and robotics. So.. what does that go with?
Advanced super tech? Tau weaponry. Battlesuits become advanced Legio Cybernetica. Riptides turn into Knight walkers.
More advanced tech and cybernetics? Necrons. I have the first of my 'Nightscythes' built out of an Ork Bommer (which looks like an IG style vehicle). I have a Techmarine with a Warscythe zooming around on a CCB. Warriors have the perfect feel for Skittari (Firewarriors did too, but I decided to go with more big robots with the Farsight rules).
Any race could technically be done this way. A friend of mine is dying to put Squats on the board. They synch up pretty well with Iron Hands rules. Maybe you wanted to run a race of Cthulu'ish giant squads... That might be a little more tricky, but I'm sure we could link up some Nids to them.
Especially with Allies, you should be able to make your own amazing race by using exactly the squads you want from two factions to make some new race with the stats you want.