As for villain team ups, that worked really well in Spidey 3.
This is diffrent though. Not going to debate if spider-man 3 was good or not just say this. Because this is a team movie it would be closer to Magneto , toad , Sabertooth and mystique teaming up. As it's not 3 vs 1 it's like 4 on 4 AND with the advantage of previous movies.
Imagine X-men 1 if it had a back story already shown on a film. You see the possibilities?
A villain team up could work.