I think they still have plenty of stories to tell, the introduction of Gwen Stacy was a good step, so you don't lose the continuity. We were already introduced to Connors in 1, showed him in 2, and now he has a main appearance in 3. I think is very likely that we'll see that arc in the 4th one.
Ok, the main story of revenge between Peter and Harry might be over, but there are stories to be told. Gwen, Connors, Jameson's son (Man-Wolf) and perhaps new introductions (Felicia Hardy?).
That's why they should have spinoffs for those characters. Tobey and Kirsten surely wouldn't want to do ANOTHER spider film if it is going to focus on minor characters. Dunst(dead or alive) will be gone after this one and Tobey will want to do other projects. So making it Spiderman 4 with a new Peter and MJ would water down what the original 3 films have accomplished.
How long do we have to see MJ and Peter say I love you and have cars thrown at them? It will get redundant and boring. A spinoff is the best outlet to tell the "other stories" but IMO this story should end here and not be dragged on or bring in MORE villains to outdue the previous film.