Seeing how Venom appeared in Spiderman 3, they should get Carnage in Spiderman 5. This won't allow too much repetition and keeps the material fresh. It's kind of like how Green Goblin appeared in the first, then in the second film, you have Harry discover his father's 'lab', and in the third one, he decides to be the new goblin. There's really no need to rush it; if I recall correctly, didn't Sony say there will be 3 more Spiderman films after the third?
Therefore, in the 4th one, they could use Lizard, since Dr. Conners is still there and have a second villain, Kraven or something.