Hey all! Thought I would weigh in on this. Probably going to get caught up in the drama, but hey, why not?
So yeah, most fan games are based mostly around the genesis sonic games, the Adventure era, or modern (boost) sonic, with slight variations. Sure, there are places where they vary drastically (and that's pretty cool), but these are the major bases.
So why is this?
Well, I think it has more to do with the medium rather than anything else. Sure, Fleetway, Archie, and the shows are more obscure than these "mainline" game titles, but I think it has more to do with the fact that these games represent established game design formulas that are easy to iterate on. When people start to think about where the gaming franchise should go in terms of gameplay, it is usually an evolution of one of these gaming styles rather than a complete overhaul, which (as far as I am concerned) is probably a good thing.
While the games are good gameplay references, the comics and shows are better story references. That's why much of the fan works stemming from these mediums are story-based (like comics and fanfiction) rather than gameplay-based (like a fan game).
I know that many Pokemon games take more from the story department of other mediums, but I think this is a by-product of the type of game. In short, an RPG is much more conducive to storytelling than an action platformer. Many times, Sonic fan game creators like to innovate on the gameplay side first, and then maybe create a story to fit. I understand your frustration with not being able to find games based on your favorite mediums, but I think there is a fundamental misunderstanding about the game design process for these games.
Besides, there is the idea that if you want to see these games so badly, you should make it yourself. That's what got all of us started here. Learn some game-dev skills, build a game, and submit whatever you come up with to SAGE. Sure, it's hard, but the process is rewarding and the computer skills you learn can be helpful in your future career, whatever it may be.
Also, in terms of needing to meet a bare minimum to be a "true" sonic fan, all I'll say is this: being a fan of anything (including Sonic) is something that you decide for yourself, not something that others can decide for you. Knowing about the more obscure Sonic mediums should only be a possible by-product of being a fan, not a requirement for it.
Thank you all for listening to my TED talk.