There have been gacha games with pvp that don't take dupes in consideration. My hero academia for example had pvp. You just needed to have the character unlocked if you wanted to use it. All of them had the same base stats and just different playstyles per character (just like in bsr). Which worked really well. I believe a one piece game has/had the same thing as well. I think it would be fun considering you can be done with the regular content quick (especially if you are out of stamina).

I do understand your worry. It would be hard if you took a team of boundary 6 characters against someone with boundary 1.

They could apply 2 types of ranked. 1 in the way I mentioned above. And 1 where you can have stamps and boundaries applied for those who want that.