Is there anything in the fluff that has a presence for these races outside of the milky-way galaxy?
I'm thinking of a post Imperium campaign where the PC's have been sent off to begin again in the Andromeda galaxy (the journey will take hundreds if not thousands of years while the PC's are in stasis).
I would expect to find tyranids (maybe not too many without the Astronomicon to draw them in) but I wondered if Eldar/Necrons might be there? Or perhaps other races raised up by the Old Ones long ago...