Bit late, but I’d suggest studying good game design. I’ll link some Sonic-related material that I think anyone who wants to make Sonic levels is pretty much required to read/watch:
Sonic Spitball, Part 1 - Gameplay analysis:
Science of Sonic, a website that picks apart the games of the franchise:
http://www.smile.talktalk.net/sj_index/nss/nss_contents/index.htm
Sonic level design thread on Sonic Retro:
http://forums.sonicretro.org/index.php?showtopic=28302
Christian Whitehead/Taxman interview:
https://www.gamasutra.com/view/news...od_Sonic_level_design_is_all_about_slopes.php (this is only a snippet, the full interview is linked at the end of the article)
Sonic maps:
http://www.soniczone0.com/
I could look up some more material, but this is all that came to mind. Hope it’ll help you!
I only partially agree: while undoubtebly uniqueness will make your game stand out from the rest, it’s not the only factor in its success. For example, Super Mario 3D Land is very fun, even if its level tropes are retreads of older games, but because of that, it’s very easy to let it slip under your radar. I would say the key elements for a succesful game are both uniqueness and level design.
One last tip I’d give you is not to plan your level around a gimmick that is too, uh, “gimmicky”. Wacky Workbench, I’m looking at you. I hate you, Wacky Workbench.
Aside from that, go crazy. Good luck!