This will begin around 112,000 AE or after Earth for those returning and as for the antagonist I was going more along the lines of a sect of people who are no longer exactly human who remained on the surface. 

They will have left Earth around 2100 BC or so. By this point in time most major cities and metropolitan areas will be completely colasped and there will have been massive ecological and economic damage by this point. The political systems are mostly helpless by this point after many many natural disasters and droughts. Earth is simply incapable of holding the sheer number of people. There is sickness that is rampant and not enough resources. 

There will be small hints of sci fi more or less just more advanced technologies and such. Think like the technology in the book The Host by Stephanie Meyer that's briefly described there. Highly advanced medicines. 

Earth by this point will be completely restored to pre human stats. I have a whole set of creatures I'll give you guys so we can interact and I'll be using a sort of dungeon kinda generator thingy that will guide those interactions so it's not just me. I will also be appointing at least 1 co gm maybe 2 since I'm busy on the weekends usually. Once I get home tonight I will put up and OOC and we can continue discussion there and work out some more details about things