I have a few, but I think that my most unpopular opinion about Sonic is that we don't need Sonic Adventure 3. I think that Sonic Adventure and Sonic adventure 2 were good enough to be two of the greatest games of the franchise, and making a new game and calling it "Sonic Adventure 3" would ruin the name of Sonic Adventure if it's not up to par with both of the previous games. Plus, in my opinion, the adventure games had their third game with Sonic '06, since the game mechanics and storytelling (not story) are very similar to both Sonic Adventure and Sonic Adventure 2. If Sonic '06 was actually a good game, I'm sure people would say it's technically Sonic Adventure 3.
(Picture by: ResidentEvilffs)