I absolutely loved being able to respawn bosses in DS2 and I heard you could also do it in Sekiro. I think I remember seeing a total conversion mod for DS3 which let you respawn bosses from statues in Ludex Gundyr's arena, so it should be possible, but I just don't have the experience with From games to try and do this myself.
If anyone manages this that would be seriously awesome, a big thank you in advance.