That's going to hold back alot of suggestions. I think all the big and active ones have been mentioned already -- SWTOR, EVE, Wildstar(I don't this has much population anyway), Defiance(no clue on population).
There was Firefall too, but I have no idea if that's still alive/populated. It wasn't doing well last I heard.
If you don't mind paying and playing Alpha, you could get into something like: Project Genom which has ongoing testing. I have no idea about how populated it is, or how much content it has, personally. You could check their forum and find out.
All the other Sci-Fi MMOs I can think of are pretty low population and pretty old.
Comments
Godfred's Tomb Trailer: https://youtu.be/-nsXGddj_4w
Original Skyrim: https://www.nexusmods.com/skyrim/mods/109547
Serph toze kindly has started a walk-through. https://youtu.be/UIelCK-lldo
There was Firefall too, but I have no idea if that's still alive/populated. It wasn't doing well last I heard.
If you don't mind paying and playing Alpha, you could get into something like:
Project Genom which has ongoing testing. I have no idea about how populated it is, or how much content it has, personally. You could check their forum and find out.
All the other Sci-Fi MMOs I can think of are pretty low population and pretty old.