Okay. Hear me out. You see those tiny group of slimes that usually spawn. "This is going to be easy" you say. But the moment you approach they seem to bounce towards each other and GLORP. They merge into a medium slime!

Merging
When tiny slimes see a target, instead of pursuing, they first check for a total of 3 slimes of the same type and size in a small radius(configurable) and if 5 seconds pass(configurable), they will group together and merge into a medium slime. Any items they were holding, the merged slime will keep or it will drop 2 to keep 1 at random. And after 10 secs(configurable) if 3 medium slimes spot each other they also merge into a giant slime.
To prevent exploit of infinite slime and exp, the merged slimes should be considered a separate entity or just have a tag that prevents them from dropping anything other than the item they have absorbed. Merged will only drop exp or slime when the tiny versions are slain.

I already had a rough concept of slime merging in my backlog. You add some really important considerations that I hadn't thought about yet. I might have to reconsider part of the way I envisioned it. Infinite items / XP is dangerous.

It's on the same level as "slimes no longer split when defeated, but just shrink in size and slice of smaller slimes"