Just about everybody agrees that RAB is Regulus Black, including me. What I haven’t heard anyone say before, but that I think might be quite possible, is:
Regulus Black isn’t dead!
JK Rowling has just “casually” mentioned the Draught of Living Death a few too many times for it not to be used, going all the way back to Harry’s first potions lesson. SOMEBODY is not really dead.
It’s not Sirius. We saw him go through the veil, so he’s dead. Cedric is really dead. Harry’s parents are really dead. And JK Rowling has confirmed Dumbledore is really dead. Who’s left that could be faking? RAB.
Nobody saw RAB die; even Sirius didn’t really know how it happened, but speculated it wasn’t Voldemort.
The message Harry found in the locket said something along the lines of “by the time you find this I’ll be dead” - could mean RAB was expecting to be killed, but could also be a cover.
In nearly every book there’s a character that isn’t what they seem on the good/bad scale; I think Regulus will reappear and turn out to be good, or at least good enough to want Voldy gone.
Of course it could turn out to be someone in book 7 “dies” but comes back (Malfoy?), but I’m betting on RAB.
If Sirius had written a will he could, presumably, cut his brother out of it. IIRC Harry was in Sirius’s will as his sole beneficiary, so even if Regulus was alive Kreacher would still have gone to Harry.