I'm surprised that no one has made a mod based on a concept of what could be chasing the vasari.  Yes yes I know "but we dont know and if we try we may ruin it and piss everyone off", well to that I say it obviously isn't what is chasing them and it is just a possible representation to spice things up a bit without adding other entities into the sins galaxy.  Possible ideas is this

1.  Throw everyone's ideas ive seen into the mix, phase gate theory, nanite theory, make it a demonic essence that took control of nanites to grant them a physical appearance (make player portraits look like grievous or that new ultron or whatever) and they assimilate everything, machine/bio/what have you.

2.  Make their colonization unique, don't make them playable right off make them have to be defeated or something, as they take planets on the map the phase lanes go blank and you just see black where the planets used to be, a staple of this could be defending and probably losing as they attack your planets almost forcing alliances to repel them (possibly make a tech that makes ships able to phase jump to Nameless territory that lost planets and traditional phase lanes due to some special ability said nameless has)

3.  Make their ships a hybrid of everything, once they are playable make them learn new ship "schematics" due to devouring the Tec/advent fleets and new Vasari, make what comes out some nanite craziness (like say more alien like such as vorastra or warp the Kol into some demonic representation manifested through altered tech and nanites and corrupted unity).  I'd love to draw some concepts for kinds of ships like that BTW 

4.  Make them immune to being seen by the unity or something since it is potentially corrupting them make any culture that goes into a Nameless territory disappear and I dunno have some more creative ideas on it.

5.  Gotta do something with phase inhibitors, like say if they assault a planet make them take control of it and use them to inhibit all none Nameless ship speed/construction speed/anything since it could be their tech.

6. "That makes them so OP, it'd break the game if you played against them/with them!". Well duh dummy thats why you add new techs to all other races, you could put an emphasis on adding more alliance options to band together against this threat (Vasari sharing nanite tech to help battle nanite, etc.) And do all kinds of stuff.

7.  Let the comment rage fest commence, but seriously if someone would do this I would deff go crazy on ship/structure/design of this kind of new race to substitute for a fake Vasari hunter until we find out what it really is.