Awesome idea! What about having elemental temples in the slugterra dimension where you can find the elementals! They could use the medium method of catching. With special powers?
Do you have crafting recipes?
Here are some ideas that I hope will be helpful:
Boss battle against doctor Black and the emperor.
Black drops lots of slugs and his machine gun slug cannon, emperor drops his sword that is also a slug slinger, works as both sword and blaster.
Also maybe add in special blasters that can be found in dungeon chests (like the double barrel blaster that combines slugs) and a way to upgrade blasters so the shoot out more powerful slugs.