#1 What units in other mods does this mod lack?
have to agree with krronus, if you added more FoK stuff, maybe replace units models and whatnot. Oh, and add the swooping hawks...they rock. and harlequins from SS Legions mod. They rock too 
#2 What should I add to the mod?
yup new fxs are a must. And maybe consider to split certain races into sub-races.
I'll drop some ideas anyhow 
Ideas
Chaos :
mark specific sub-races. choose your mark during the start of the game and only be able to build that specific mark's unit (plague-marines, plague-bearers, GUO for Nurgle, etc.)
Space Marines :
divide them into different deployments, like the SS Legions mod. You get to choose between the expensive and elite 1st company or the ordinary cheaper company.
Eldar :
it'd be cool for you to add a whole new sub-race for the harlequins. those morbid clowns are AWESOME
Tau :
Split into Kauyon and Mont'ka (like what Master Necro said at the Apocalypse forum)
Orks :
I couldn't careless for these brutes (or you could say I have no idea how to split them)
Sisters :
still thinking of combining them with the IDH mod....
IG :
split between mechanized (vehicle based) and infantry. They just a bit crowded with units...it gets, distracting
Necron :
okay as they are...I think
Dark Eldar ;
Good god, just add new units and you're fine
#3 Is the Complete Collection too complicated to install?
Nope, I had no problems whatsoever installing it. I find it easy actually 
#4 What are some new XP2 races to use?
Daemons, steel legion, better inquisition
#5 Would you be upset to see that 1.63 won't be released until next year?
personally, I dont mind, since as krronus stated, I'll get updates. But I'm sure the kind community wont mind us taking a bit longer to polish our mod here and there. And make it have shinier new stuff. 
@RevanFan : The team will try our best to make this mod as polished as possible and avoid any major game-breaking bugs as well as those pesky hard to spot "cosmetical" bugs