I feel like you might be writing this backwards, but okay.
Ouch. The creatures are much more useful. Especially since you can have skills based on the substance
of the creatures.
Alright then:
Since it
is (only) a Rare class, it should be stronger than common classes but not the ultimate legendary class of overpoweredness. So first let's look at the common and uncommon variants, since that'll give us some context.
At its core, slime is sticky and slippery and thick. That makes it great for crowd control and... not much else. So the lower variants would be weaker, but the same. Maybe an Ooze Mage can make sticky thick mud. A Grease Mage can make slippery mud. A Mud Mage just makes mud. :(
Now we have a little context, I suppose. Let's restrict the skills quite a bit, but have them build on our classes. A mud mage has [Mud Jet], a common skill that squirts mud at a target within range.
Ooze and Grease Mages upgrade [Mud Jet] to be their elements
and have [Pool of Ooze/Grease], an uncommon skill that summons a 20 sq ft coating of their element on the ground within range.
So going from there, a Slime Mage would have [Slime Jet], [Pool of Slime], and a Rare skill like [Viscous Coating] which coats targeted ally or enemy within range in a layer of clinging slime. Enemies affected will have their movement impaired sharply, while allies will gain a [Freedom of Movement] effect that makes it easier for them to escape grapples and binds. The slime will constantly flow towards the target, even if they try to remove it.
Alternatively to any of the above skills, you could have things like [Slime Shot], [Slime Wave], [Slime Torrent], [Slime Nova], [Slime (Weapon)], [Control Slime], [Shape Slime], [Harden Slime], [Slime Body] (AKA Flesh to Slime), [Banish Slime] (For those hard to reach messes), or [Slime Sense].
There's an interesting alternate take on the class mixed in there. Use slime body to turn into slime, then control or shape slime to control yourself, then use slime weapon and harden slime to attack and defend. So rather than a cc mage, you'd have more of a regenerator warrior. You could also go for a more direct damage build just by making the slime corrosive (though I feel it defeats the point) or you could make a healer by having the slime cleanse or mend injuries like some kind of freaky skincare lotion.
