I'm on board with Bird returning on another two-way deal, but I'm not sure there's going to be room on the roster for him short of an unexpected trade. I figure Monroe's probably gone and whoever they take at 27 gets Monroe's spot on the roster. If Baynes leaves as well they need to replace him with another big, not a swingman, but I think he'll ultimately be back. While Nader may or may not see next season, if he's gone I expect it means Ainge went hunting on the free-agent market with the mid-level for someone like Tyreke Evans.
EDIT: Forgot that Larkin could also be departing this summer, which could theoretically open a spot for Bird there, particularly if Smart and Rozier are both back and Nader (who I haven't given up on) is replaced with a ball-handling wing like Evans. I guess that's the mechanism for it happening, if it does.