What is humankind’s best case scenario?

Best case scenario for what? Survival? In the long run we will die out. If we manage to get into space sustainably (a very big if), we may last longer, but will eventually die out as well (space colonies, due to the vast scales of distance, will probably diverge from current humanity as well if they last any appreciable length of time, which is unlikely).

The universe has a time limit, and our expiration date will precede that by a very large margin.

Best case scenario: get replaced by the artificial like humanoids of Charlie Stross' book "Neptune's Brood". At least those humans could survive space travel and move life away from a single planet, which will be consumed when the sun goes red giant in a couple of billion years.

We are the Borg?