I was scrolling through the pages of the Just enough items mod.
As I was scrolling through I found the Ultra Ruin, Ultra Forest and Ultra Deep Sea keys.
It got me curious to what they could be, since I had never seen them in the game. So I came to suggest maybe adding naturally spawning Ultra ruins you have to parkour through or some sort of Redstone puzzle. Same applies for the Forest and Deep Sea as well. Maybe for the forest some sort of redstone maze or some challenge from trainers. For the Deep Sea ruins you could make some sort of water puzzle sorta like the infamous water temple from Ocarina of time and inside each of the respective puzzles there would be a chest revealing some awesome stuff inside. I think it would be really cool and interesting to see these ideas come to life. I do have ideas on how you could get these keys in the first place.
Either you could 1
Craft it with very valuable and expensive items
Acquire it from a Mega Boss and have it be at a crazy low percentage rate
Or a NPC Quest sorta like the Dynamax Quest which I think would be really cool. you could have a quest for the ruins to catch rock type or fossil pokemon. Or catch a grass legendary for the forest. Or a water type legendary for the deep sea which would be in ultra space
Thanks for reading this suggestion and I hope you enjoyed!