Since we've now seen that the showrunners have no qualms bringing back "old" characters back on screen, I would love to see some flashbacks to Boba's youth with Jango. They could use Tem in the role of Jango, of course, and use that fancy-ass de-aging CGI technology to make him look more like he did in AotC (and IIRC, Tem was de-aged using similar tech in Aquaman during Jason Mamoa's childhood scenes, and I don't think it was half-bad?).
Honestly, a young Boba in a flashback could probably be played by any little kid approximating the appearance of a younger Daniel Logan.
Given that The Mandalorian centers strongly on (surrogate) Father-Son relationships, and that Boba's legitimacy as a son to Jango has already been questioned by characters like Bo-Katan, I would love to see an emotional, down to earth scene between them. I think it could be really fascinating to see that and solidify the importance of Jango as an actual father to Boba - not a mere "donor."
-BFFC Manji aka Jess