Welome to SocialHub! Nice that you found Zappa already, which is indeed investigating use cases where your interests align. There’s ongoing user research for Zappa by Doug Belshaw and Bonfire team, and a good place to discuss is in the Bonfire and Bonfire Technical chatrooms on Matrix.
Other than that some notable research projects in the forefront of fediverse futures (and broader applicability, though not ML-related atm) are Spritely by @cwebber and DREAM where @how participates. To bootstrap you can look at Guide for new ActivityPub implementers and delightful lists at Fediverse Futures - Codeberg.org.
Some weird ideas are on Lemmy at Fediverse Futures brainstorming area and here in Fediverse Futures category.