Necrons and Tyranids won't happen.
Tyranids are hard-locked on feeding and have a collective mind, There's simply no way to add them that would make sense. Genestealers, while at least able to communicate, will still go for their main purpose of breeding, and also have brood telepathy. It's a bad idea to make PvP rpg.
The only Necrons that would be playable are Lords, Overlords and Crypteks. Of those, only Crypteks could have a reason to join RT crew. Lords/Overlords wouldn't move without their armies and fleets.