1) Totally new faction
2) New faction but had connection with ME
3) Known species faction( which means those who reach Andromeda fight among themselves )
4) Leviathan faction
5) Remaining reaper faction in dark space( if there is any )
6) Reaper faction from Milky Way( "somehow" overwrite all the choices you made )
Totally new faction, I think.
Possibly a bunch of Protheans who had the same idea that we did in escaping the milky way.
I think having any of the villains from the original trilogy is a bad idea. Please, no more reapers. Dear god, please no more reapers.
The whole point off going to Andromeda is to have a new beginning, one that the series massively needs after the debacle that was the ending of ME3.
We don't need them to be the villains in ME, they're already the villains in real life.