Love the Sinister Six, but the purpose of the Sinister Six is to do what none of them could do on their own, which is to take out Spider-man. Therefore, making Spider-man a necessary part of any story revolving around the Sinister Six. Without him they lose their purpose for teaming up and could just as easily do their evil deeds on their own. So, if it is then assumed Spider-man is required to give them purpose, then it should be equally assumed Spider-man would need to make a strong appearance within a Sinister Six movie for it to be successful. However, at that point one would have to ask why call it Sinister Six (a virtually unknown name to general audience members) when they could cash in on Spider-man with a title like Spider-man: The revenge of the Sinister Six. So, yeah I say bring on the Sinister Six for like part 3 or perhaps even 4 or 5 of the new Marvel/Sony Spider-man movies, but the idea of doing the Sinister Six as a seperate movie and possibly without Spidey, terrible idea. However, I trust in Marvel and I really don't think they would have made the deal with Sony if they felt they couldn't trust them to listen to their advice with relationship to the use of their characters. Sony knows by listening to Marvel their Spider-man franchise will bring them in the billions (at least eventually) that it always had the potential of doing, but for whatever reason seemed to evade their grasp. So, if Marvel tells Sony doing a seperate Sinister Six movie is a bad idea for everyone involved, I got to believe that Sony is going to respect their knowledge of the characters and either scrap the project or build it with Marvel's help into a future Spider-man movie. So, everyone panicing because we don't know enough about the deal details, I just really got to believe that the people over at Marvel Studios wouldn't have made the deal if they felt doing so would jeopardize their well established connected cinematic universe. What they have done so far with the MCU tells me they are very intelligent individuals with a passion for their characters, so I have every faith that Spider-man joining the MCU is a good thing for all parties involved, especially fans.
Surfer