I personally have enjoyed the Slimes beneath the ground, and the danger they present. Only problem is they are bit.....boring.
What I suggest is a new set of slimes, each with a different color pallet and behavior patterns. As well as different stats for Meat-In-Jelly and any other recipe that ends up with Gel as an ingredient.
A few Examples:
Red Slimes: Fights in groups, but doesn't aggro. Attacking one is attacking all
Light Blue Slimes: Passive slime, won't aggro or link. Gets stronger as its HP gets lower.
Yellow Slimes: Instead of spawning a group of slimes, One Big slime is spawned. As you cut through its Hp it starts to split into smaller HP targets, with less attack.
Dark Blue: Spawns around water, Always aggros and Links.
Black Slimes: Aggressive when in the dark, however they run if they see a torch, lit mining helm, or fire.
I'd also like to suggest a few new recipes to go along with it.
Gelatto:
3 Snow
3 Gel
3 Nuts or Berries
Nut Butter(Quern):
1 Jar
4 Nuts
1 Rendered Fat or Butter
Jelly(Boiling Cauldron):
1 Jar
1 Gel
3 Berries
Edit:
Nutbutter and Jelly Sandwich:
1 Bread
.01L Jelly
.01L Nutbutter
I admit I don't know what stats are really needed in the food chain, I just thought more food and things to craft with the Gel would be fun. Along with more dangers in the underground.