Ya it’s better to create some original work first but I guess you could always try LEGO Ideas. A lot of fan creations have been turned into real sets.
I’d love to create something for Sonic. I’d do it for free

I don’t care about royalties or anything. I don’t believe I should own something I’m specifically making for another franchise. I’d be making it for Sega.
I think the Green Hill Zone theme and others (excluding MJ and Crush 40) are actually owned by someone else. Crazy right? It’s why they’re not in the movies. Well there’s the brief jingle and Emerald Hill was only in the teaser.