Copiers and stealers in particular can be made interesting in what they say about both the target and the user. Do they take one power from a brute and show how effective it is when used with finesse? Do they take a power that's been used for traps and trickery and use it for direct combat? Can they come up with combinations that no one else has thought of? Things like that.
I brought up the character All for One from My Hero Academia as a good example of how interesting limitations can lead to interesting workarounds in the matter of stealing and replicating powers from others.
All for One isn't limited in quite the same way, and for why that is will require a bit of a digression into how powers work in that world. The powers, or "quirks", are as much a part of their user's biology as they are superpowers. They require training to strengthen, and they exact a toll, such as nausea, injury, fatigue, etc., when used. As a power-stealer, AfO gets a lot of synergy and flexibility, but he doesn't really have the time to strengthen his quirks past the point they're at when he steals them and is struck with the drawbacks of each of them. Still immensely powerful, but a lot more interesting than a Sylar-esque "I win" button.
For wanting to explore another focus to power copying and talking about the devil, why are Sylar and Peter from Heroes so powerful? Because they were made to represent the Biological ideas of the Mosaic and the Chimera respectively.
Peter's original ability - Empathic Mimicry is by far the most powerful ability in the series - re-sequencing the user's DNA to match the genetic signature of nearby EVOs. this allowed for the automatic acquisition and permanent storage of numerous powers. Peter's variant was unique in the sense that it was originally based on his emotional empathy.
Gabriel Gray/Sylar has his own power; Intuitive Aptitude - the ability to instantaneously understand the inner workings and functions of any observable system (biological, technological, or informational).
In Peter's case, Mohinder even states this in the Season 1, episode "The Fix" stating: "Peter’s specific DNA allows for a blend. Like colors in a MOSAIC, re-sequencing itself to mimic the abilities of those around him."
In Biology, Mosaics and Chimeras are organisms that carry extra genetic variation. The difference being the method.
In the case of the Real-Life Mosaics, their genetic variability is more self-imposed; the result of a zygote (fertilized egg) that undergoes mutational/chromosomal alterations from its initial DNA. This is important, as it states that the Mosaic is able to induce genetic mutations in itself to achieve added variability.
The Chimera is different. In Real-Life, Chimera's are formed when two or more different zygotes forcefully fuse into one composite, each with their own variability integrated into the new whole. This implies that Sylar uses his power to forcefully graft foreign DNA onto his genome like a Chimera, essentially making him a Biological Frankenstein. This is different from Peter who's DNA is the same throughout, but alters ITSELF in order to create new mutations.
Noah Bennett himself states this in the Season 1, episode "Fallout"; When talking to a captured Sylar in his cell he says: "I think you're insane. I think the infusion of so many alterations to your DNA has corrupted your mind - all this power is degrading you."
In English
Peter's DNA is very unique, one-of-a-kind. It changes itself to learn and memorize new sequences (genetic templates for powers) and stores said sequences. When Peter calls on one power, his own DNA remembers the sequencing and structure of said power on a genetic level and uses that to recall the power, because Peter's DNA is essentially capable of shapeshifting.
Sylar's DNA is plain and normal like everybody else's. He uses his Intuitive Aptitude to understand the function of an EVO's powers. He then samples the EVO's DNA via the brain (as that's also where he can see the neural impulses or instructions to use said power). He then forcefully integrates the DNA into his own genome, damaging it in the process, with his mental instability being a side effect.
However, the question of how to balance a power stealing ability in less specific settings remains open. One option may be to establish it as a standard skill; if anyone can cast a spell / technological device of mimicry, then it becomes a tool rather than a cheap gimmick, and, again, its particular usage could be useful for character development as well. In the case power stealing is particular to few individuals, it can be rendered less overpowered by exploring how human memory actually works. Depending on which model you consult, there are more than enough memory types and even subtypes, so let's clear it up a little and stick to those that are ubiquitous in most hypotheses: short term, long term, and the latter's subdivisions.
A power-copying skill may grant its caster access to a nearly unlimited repertoire of techniques and skills, however, its necessary drawback could be that it's transient. The caster don't learn the copied ability, they instead just retain them momentarily in their short term memory; just enough to perform the ability and then forget it. They don't remember nor comprehend how to use it and how it worked afterward, yet they may be still able to analyze their opponents beforehand and exploit that (learned) knowledge when they finally replicate their enemy's power.
