That said, today I did come up with an idea from a dream. I realize the idea of a "Slime Girl" is ridiculously overdone for a fantasy setting, but what about for a superhero setting? (Okay even there it's probably been done...) In this case, the specific concept I came up with is a Slime-Succubus hybrid.
In this universe, slimes have all of the abilities you'd expect them to have. Extensive shapeshifting, absorbing, being able to make lesser copies, special sight, being able to dissolve into a pile of goo, the goo moving, "metal slime" skin, you get the idea. But one thing slimes can't really do is have control over the pigment of their, for lack of a better word, "skin". They can change it somewhat, but they don't have fine control over it, in spite of being otherwise excellent shapeshifters.
In this universe, succubii have the opposite problem: they are the absolute masters at superficial shapeshifting. They can change pigments of their skin tone, they can change things like how much fat is where, make themselves a few inches taller or shorter, a little wider or thinner, so they can basically masquerade as essentially any human (or "human"), but can't go into obviously nonhuman shapes, and can't go too big or too small. They can't become too outside of the "norm". They still need two legs, two arms, a central mass, a head, etc. Their basic shape stays the same, even if the specifics of it, they are the masters of change.
So you can see where this is headed--the Slime Girl I have in mind as a Slime-Succubus hybrid inherited both traits. The extensive shapeshifting of the slime and the extensive form changing of a succubus (or would it be extensive form changing of a slime and extensive shapeshifting of a succubus? You get the idea tho).
So she can pass as human, or as a default succubus (succubii have horns, wings, and a tail by default--various succubii have different types of these, from cat horns to ram horns, cat tails to devil tails, but always have them; obviously, succubii can hide them in their shapeshifting as their wings/horns/tails are where the majority of their shapeshifting mass comes from), but has all the powers of a slime.
Being a liquid, no need for sleep (and much lower needs for other things), having things pass through or be absorbed inside of her, store things inside of her body, immunity to physical damage in multiple ways (metal slime armor, extreme maneuverability to avoid being hit, even when hit being a liquid so blows being absorbed the way liquid would, not really having vital spots to wound so the wound not being critical, etc.), immunity to blunt impact, having an adhesive body that sticks to things and can have things stick to her, immunity to poison and disease (racial slime traits), immunity to mind control (shared racial traits between succubii and slime, with between the two covering literally every possible form of altered state of mind, with the singular exception of getting drunk, which both are actually more vulnerable to...), being able to eat/absorb anything, being able to tell what's poisonous (among other things), recognizing deception (this one's a succubus one), having the ability to read social cues and be a universal translator (also succubus), being able to generate poison (including paralyzing toxins), having the power to seduce (albeit, thanks to her morals, refusing to use it), being able to absorb any liquid (including water, natch), changing size/body proportions proportional to how much mass she has available (and she can gain/store mass from water in excess of what she has already), filtering water, having healing powers from her water, being able to unleash water with limited hydrokinesis (water connected to her body she has full control over, water she shoots she can push around or even make into a stream sharp enough to cut/pierce, but finer control is lost if she's not actively in contact with the water), control over whether what she envelops is digested or isn't digested, and for that matter, whether to drown them or not drown them, mastery of how solid she is, what texture she is, ability to control clothing, use of extra limbs (an endless amount of tentacles) that have incredible fine motor control, unmatched sense of taste, touch, and hearing, ability to mimic voices perfectly, perfect memory, limited tactile-based telepathy (feeling the surface-level thoughts of those she contacts), passive ability to absorb through this method memories and feelings (empathy basically), genius-level intellect, being an incredibly fast learner, especially skilled at navigating the human world (averting a typical slime girl stereotype), skilled at acting, no need to breathe, ability to create weapons from the body (blade-arms, blade-tentacles, axes, knives, you name it, shapeshifting + metal = anything can be a weapon), an ability to quickly learn the fighting styles of others and incorporate them into her own (and find counters to them), incredibly strong self-regeneration if she somehow ever does get hurt, ability to generate acid (and immunity to it), being able to absorb/create liquid metal, having the required heat resistance of this, ability to (with enough time) shift "types" of slime to gain immunity to cold or heat (not both at once obv, and it does take time, extensive time, to mold herself into that different "form" of slime), being able to basically act like rubber, capable of being either a conductor or insulator of electricity (and to a very limited extent, for magic in general), through being liquid being able to melt through surfaces, pass through them, slipping through even the smallest of cracks, mastery of dexterity/nimbleness/hand-eye coordination, superhuman strength, increased speed, flight, ability to mind control others (not used) through specialized created slime cores, elasticity, a limited hammerspace for objects/mass, a form of farsight from basically having eyes that can become a magnifying scope, ability to see details from it becoming a microscope, forming doppelgangers (other slimes controlled by the main body too, albeit with one mind so it being laborious), ability to sense water of all forms from an incredibly long distance, and with incredible focus, ability to track people within that range (because people have water) (but this is not an easy ability to use and requires intense focus to maintain the clairvoyance this entails), the ability to "see" everything around without eyes in a sight-without-sight in a 10-yard radius (which also lets her see magic itself), and limited skill absorbsion from things absorbed.
From these skills, she's mastered every form of weapon, and she is quite good at rune-based magic (drawing magic, basically). Her ability to cast non-rune magic is nonexistent tho, but she still has extensive knowledge of it, and high ability to counter it (in part due to being able to see magic itself, in part due to an a limited form of adaptive defense, but also due to genius analytical skills and being able to, so to speak, predict magic--not a proper power, but effectively one thanks to her instant calculation time since she's able to do basically any math/physics/etc. problem in her head instantly).
Sadly, this is all I came up with. Abilities, and loosely ones at that, and the concept being superhero rather than fantasy adventurer. (I might look into more powers, too.)