It's hard for me to say what villains I would like to see, because we don't know how big of a role each already casted character will have. But I do know that I don't think Lex can be a standalone villain, not with Eisenburg being Lex. It's just incredibly hard for me to see Eisenburg pulling this role off. So I definitely think they should add another villain, to take some of that pressure off.
It's also hard for me to say which villains I want to see, without knowing how many movies this franchise will put out. For example, how many Justice League movies will we see? Will there be a Batman solo movie(s)? Etc. Because I'd like to see them hold off on Darkseid and Doomsday, I think one of them should be the final villain of the Justice League (my choice being Doomsday for the final villain). At some point I would like to see The Joker, but that might be too soon for this movie, particularly if we get more than one Justice League movie and at least one Batman solo film.
So for now my choice would probably be Lex and Metallo. If they have to add a 3rd I would go with Brainiac, because again, I'd like to see them hold off on Doomsday, Darkseid and The Joker. But if we are only getting 2-3 movies, then they should just jump into one of those 3 with Lex now.
Hard to say without having a layout. I'd like to see a bunch of movies, (2 or 3 JLA movies, at least 1 Batman movie, and 1 solo each for Flash, Green Lantern, Aquaman and Wonder Woman).