I believe the Han/Fett rivalry was set up in the (no longer canon) Han Solo Trilogy by A. C. Crispin. I don’t hate the idea of bringing that back, but you’re definitely correct to say it currently has no basis in canon material.

Anyway, I’d like to see the “Boba Fett movie” actually turn out to be something like The Good, The Bad, & The Ugly, where he’s a major player but not the sole focus. There’d definitely be room for Alden’s Han and/or Donald’s Lando in something like that.