I'm not certain that this question is strictly on topic but I'll take a bat at it anyway. The demon king is either a threat or not, if he's a threat then he's worth fighting, if not then another confrontation is a pointless aggravation. This doesn't mean you can't have the structure you're are proposing it just means that at some point in the epilogue he, or maybe his new minions, has to pose a legitimate threat or there's no reason to go another round in the first place.

I'm not certain that this question is strictly on topic but I'll take a bat at it anyway. The demon king is either a threat or not, if he's a threat then he's worth fighting, if not then another confrontation is a pointless aggravation. This doesn't mean you can't have the structure you're are proposing it just means that at some point in the epilogue he, or maybe his new minions, has to pose an active, legitimate threat or there's no reason to go another round in the first place.