Ah don't get me wrong, I love Necrons. I think they're cool and a race which everbody should be afraid of and who can kick anyone's ass in 40k. But, when i look at their current codex and fluff, i can't not to cringe every time I see it.
As for what people said about Rak'gol, It's a cool idea and I'd like to see it, but i don't think it's feasible. Firstly you'd need to have them come closer inside of Imperium's territory because every race in 40k needs to have some access to one another, otherwise they're too isolated and cannot interact in the fluff. And since Rak'gol's territory is mostly in Halo stars, Imperium can't invade there effectively, unless Rak'Gol themselves launch a massive attack on the imperium, which given they're raiders is not really likely, and is questionable whether they have numbers to do it.
They would also need to be massively expanded upon as they have like, what, 5 different variants of troops? And no vehicles basically. Though spacecraft and some moderate lore is already there.