I'd like to make this mod more interesting for a local world, maybe like Pokemon Legends Arceus? . In my opinion, we don't have challenges in single player (or They are not introduced naturally to players).
What if you change the way a player complete de pokedex?
Maybe creating some quest line or player skill experience could be more attractive.
Creating titles like 'Pokemon ranger' for an advance exploration and catching at least 1 pokemon in [n] important biomes. 'Sinnoh Master' for completing Sinnoh dex. 'Grass gym leader' for getting [n] different grass type pokemon.
Creating a sidemod or including a map in the main mod for searching pokemon spawns at different biome, daytime and weather. Connecting with pokedex would be great too. It's obvious, as minecraft is a dynamic map, not static like pokemon games, It should be necessary to re-explore this idea.
It would also be interesting if each pokemon entry requeries more battles and observation that just 1. Increase an specific pokemon shiny rate (x3) could be a great reward for completing its entry.
New 3 skills for different works
For completing the pokedex, exploring biomes, defeating bosses.
For crafting and mining.
Gyms, a future league? For multiplayer and servers? PVP.
Connect this experience. For example, if you want to craft a master ball you need to be 30lvl in Pokemon Ranger and 50lvl in Pokemon Engineer. Something like this.
I'll edit this post and add more ideas in the future :D.
PDTA: We really need more structures for legendary pokemon