As much as I would love to play as a necron or tyranid it would be incredibly hard to make them work and most likely quite lore breaking as well. As people have said the tyranids swarm so you'd need to play a tyrant, and even then I think they get orders from the swarm, and of course for RP purposes they wouldn't be able to say anything so it would be quite boring to RP them. Necrons again they are mindless robots for the most part only the leaders perhaps having remained alive enough to not forgot who they are so again you'd end up having tons of leaders or else necron soldiers acting like individuals which would be weird. Though I GUESS some necrons could act sort of normal as they seem to lose their memory after dying a ton of times so a newly awakened world filled with necrons that hadn't died much could have more normal ones.. but thats normal for the necrontyr and I'm not sure if after 60 million years of sleep they would be sane. People who know more of necron lore would need to input more on this Hmm reading up on necrons and they could be sort of interesting as it seems their silent king is heading back to awaken the tomb worlds to unite against the tyranids, tyranids though would be best as npc race though still.