Of course I'm unaware of all new game + features like most players. However with bosses being stated to be the progression through and to new game + I think its important that the player has an actual experience through these new game stages. I feel like this experience can be cheesed, or sped up, if the player is able to bring boss items from a previous new game + stage with them in a useable manner. The best solution I can think for this is to (preferably) add a new item that is attached to each boss spawn items recipes. This both prevents the transfer of boss items into higher new game + stages, and makes it so they aren't as trivial to craft.
