it really is one of the primary possible advances left with firearms

I truly do not believe this. There are still more inventions to be invented. From minor ones like polycarbonate front-sight hoods/wings to major ones like shape-memory alloy projectiles whose aerodynamics change based on what phase of flight they're in.

This is an incredibly partial list. What about bullets that you strike in the front instead of the back to fire them? You could make a bullpup without a trigger bar then. What about the neglected black powder? Like has there been one new black-powder gun design since the invention of smokeless powder? Look at titanium, look at lithium-aluminium, look at regular aluminium, can they handle pressure as well as steel? No or don't-know (depending on which metal), but black powder guns are low-pressure, so maybe we can have a multi-barrel thing now (as is needed with black powder due to fouling) that you can just pick up and carry? Maybe that's what Gatling would have made if the materials had been there at the time? (And titanium is corrosion-resistant.)

Then there's the even more neglected any other propellant. There is a crying need for a system that can run on heterogeneous propellants, like flex fuel in cars.

Then there's caseless, which hasn't happened yet in small arms, but it needs to and someday will.

Guns that work on other planetary bodies WILL be designed and they're going to be like nothing else we've seen.

Maybe electroactive polymers could bridge cylinder gap? I don't know; it depends on how fast they grow and shrink, what the time constraints are, and how they handle heat and pressure. And no, cylinder gap isn't just a revolver thing.

Oh, and then there's zero-tolerance machining that's a thing now. That could enable whole new types of firearm. Things you can't do because of cylinder gap.

So yeah, it might seem like all firearms design these days is just fine-tuning already-existing body plans, but I see no reason it has to stay that way in perpetuity, or why electrics would be the only thing that would be different.

How do you power the system. A battery is cool, but that means you have to keep that topped up

Watch batteries last forever, and yes, watches are high-voltage. If it has an electroluminescent display, there's tens of volts. Maybe 50.

I would have to imagine a thermocouple would be useful, but they are not efficient.

Just use the recoil energy...