Both NextSTEP and BeOS were around when Commodore went AGA so either one could have been used with the AGA chips. I choose BeOS.
Next STEP was written for 68k hardware, BeOS for RISC. There is an unfinished port of Haiku to Aranym so you may get your wish eventually.
BeOS did not exist until well after CBM went bust and a decade after Tramiel wanted to buy Amiga.