I do not see anyone on the horizon for many years, by which time, Horford and Hayward might be off the books but most likely replaced with Brown and Tatum on max contracts(the type that start at around $27 million).
The next hope could revolve around that Memphis pick. If that pick doesn't convey until unprotected and Memphis sucks, that pick could be the way to get the next superstar. Danny could use it to draft a superstar, if the pick somehow ends up top four and there is that type talent in that draft. Or they could trade it for a disgruntled star.
Who could be that star in two years? Karl Anthony Towns might get sick of losing in Minnesota. Maybe Paul George in OKC. Not sure who else I see in a possible bad situation that would make sense for Boston. Even George is a stretch. I just don't see Giannis, Harden, Durant, Curry, Thompson, or really any superstar potential player on a rookie contract becoming available then