Since the Space Wolves are already in my vote goes to the Raven Guard. I'm honestly a bit surprised how popular the Black Templars are, I just don't see them that often in the lore that I get beyond a few times in the comics.
I know why they should be fluff wise this almost screams for them, but that wasn't what I talking about.
We already have the Dark Angels as a knightly order, we have the Space Wolves and Blood Angels as presumed melee specialists. I'd personally like to see the Imperial Fists as ranged or explosives specialists, especially as their most mortal enemies will be in the game: the Iron Warriors.
I just realised, If this game has Psykers as a playable class, then it means Black Templars will not be in it. Reason: Black Templars do not use Psykers at all aside from on their ships for navigation. That's it.
then make it so that black templars are the only faction unable to use them and instead give them access to the land raider crusader as a chapter specific vehicle