I would love to see a category with all the mainline Sonic games.
The only problem is how we define what's mainline and what isn't.
These are the games I would consider mainline:
-Sonic 1

-Sonic 2

-Sonic CD

-Sonic 3

-Sonic & Knuckles

-Sonic Adventure

-Sonic Adventure 2

-Sonic Heroes

-Shadow the Hedgehog ???

-Sonic 2006 ???

-Sonic and the Secret Rings ???

-Sonic Unleashed

-Sonic and the Black Knight ???

-Sonic Colors

-Sonic Generations

-Sonic Lost World

-Sonic Mania

-Sonic Forces

that would be sick i would definitely be the first to run that 👍

Do the run and if there is enough interest in it then a category can be made is usually how it goes

anything that made it into generations is generally considered mainstream, so unfortunately, shadow is out

storybook titles are also out because of this

(edit: oh, CD is also out lol)

Lost World, Mania, and Forces lookin good

Argick type category beat lol

While on paper this could be a fun idea (barring the whole "what is and isn't mainline argument"), the issue with that is that there is no ability to future proof for future entries. There will still be more Sonic games down the pipeline and creating a category now that only includes what is out and available now is kinda silly imo. Tacking on more games in the future to a category is just not feasible either.
Again, to what Tailsdk said, feel free to run what you want, and maybe if there is more interest from others, then it may be considered by the other mods (this is just my opinion alone).
I would definitely suggest maybe looking at Argick's run of doing a "Generations Medley" which was just a marathon run of all the games that had stages/rival fights in the game.
This included
- S1, S2, S3&K, CD (per the Metal Sonic fight)
- Sonic Adventure 1 & 2, Heroes
- 06, Unleashed, Colors

the mainline games are probably

sonic 1
sonic 2
sonic 3 & knuckles
sonic adventure
sonic adventure 2
sonic heroes
sonic 06
sonic unleashed
sonic colours (ultimate allowed)
sonic generations
sonic lost world
sonic mania (plus allowed)
sonic forces
sonic frontiers?