Im pleased FS is doing 40k version of Vermintide 2…and i have no doubt game will be a masterpiece.
However, I dream of v2 style, survival game where group of survivors (civilian or military) must survive into planet isolated by Tyranid Hive Fleet… chaos is nice yes, but what brings more survival instinct into players than Tyranids ? 
I don’t think they will put Tyranid, as they are not really allied with chaos.
Chaos might ally with the Imperium to face the Tyranid threat (as they are not useful to the Great Game)
If the game is successful I can imagine we’ll see new enemy types like they tried with VT2. As to whether it’ll stick with chaos or introduce other factions for new enemies, that’s up in the air. I’d hazard a guess that we’ll see new chaos enemies though rather than other factions as its easier to add piecemeal a few enemies at a time rather than whole factions. But I’d love to be pleasantly surprised.