In addition, what about adding the Elite 4 to the game as a sort of "endgame"? You could give them their own dimension (Roughly the size of the pre 1.9 End dimension) that had the Elite 4 building, four NPC trainers with stronger-than-gym-leader pokemon, and a Champion NPC (Possibly another use for the Rival NPC) with the strongest possible NPC pokemon (That you could only battle after beating the 4 Elite 4 NPCs). Accessing the dimension could be possible through an End-Portal-like structure that required x number of badge items from the Gym Leader NPCs.
This could be a fun little feature to add to enhance both single-player and multiplayer experiences in the mod, as honestly, it feels a little lackluster and no real sense of progression after a certain point.