I found Steven in, of all places, a Thomas the Tank Engine game on Roblox. He was standing on a station platform as a little Easter egg.

He looks so goofy because they tried to model him in the same style as the static human figurines from the original Thomas show (this is what they look like).

Anyways, here's some more screenshots.

Henry: "I'm not telling him that gemstone of his is a diamond. He'll figure that out for himself eventually."

Oh yeah, Peter Griffin is here too (he won't stop singing Surfin' Bird).

Holy crap, it's William Afton. He looks like he's about to get stabbed in the back by Mabel Pines (you can also see Dipper on the station platform holding Journal 3).

Yeah, this game is pretty silly.

@EeveeTheAdventurer Pinging you because why not (and Peter Griffin).