There's a great chance Spiderman 4-6, if done, could have very great storylines.
Spiderman 4, could go like this. Venom is defeated in SM3, but not killed. He is defeated and the symboite escapes and runs off (seemingly). Brock is put in prision. The main drama would be Conners accidently becoming the Lizard and terrorizing the city.
Kraven The Hunter comes from Russia to escape certain aspects of his life, and learns of this Lizard and tries to hunt it. Spiderman must not stop both, Kraven from killing Conners, and Conner from killing everyone and himself.
Brock is in a cell with Cletuis Kassidy. The symboite has left some behind in Brock and it attaches to Kassidy, Carnage is born and escapes.
Now Spidey has double the trouble, and Kraven has three trophy's to collect, A Lizard, A spider and an alien. Venom would again reappear at the end of this movie.
Spiderman 5 could focus on an entirely new plotline involving Max Dillin "Electro" and his evil business partner Allistar Smythe. Spidery could be up aganist Electro, Smythe and his vicious Spider-Slayers.
Spiderman 6 could go alot of ways. Sinister Six....perhaps Kingpin is brought in finally and has a whole band of villians after Spidey.
I would definitly have Chamelion in this one if it is the final movie of the franchise, he could change into all the previous villians and such. Mysterio in this one too for that matter, all the illusions.
There are room for more great movies...just depends on the cast, director, money and sucess of the third movie.