I made a joke suggestion for ex bosses in off-topic, but this seems like a much better, more reasonable, and more thought out version of that. And far more balanced lmao.
Though it could take development time away from new content, there’s no real reason not to add this. It’s optional, challenging, rewarding, and increases replayability. Cheese prevention is a valid concern though, mainly with argos since you can kill him by trapping him in the room he burst out of when you first fight him.
Also, should there be exclusive drops for defeating these bosses? (i.e Lions Halberd → Mirrored Lion’s Halberd) Or just an increased drop rate for the already existing ones?