Most of mine aren't even to the design phase yet. They're set up in a story on the borders of going from today, to sci-fi. The first was the idea of using porting and such along with heavy recoils systems to make not a "recoilless" gun, but a "managed recoil weapon system"(complete with fancy military name). A balance between a traditional recoil system and a recoilless gun that uses a smaller cartridge(more ammo capacity and efficient powder use) but lower recoil. It was intended for weapons for first generation power armor and mecha--full power shot, but recoil managed to the point where the lighter firing points could handle it. Then I saw a video of that "high impulse weapon". Similar idea, though since I was going with armor, I wasn't concerned with low blast effects, just cartridge size and recoil.
More recently, I was looking at rail guns(high round count guns), I just need to find someone to talk about patents with. It's possible that the idea is already out there, just classified so I can't find it on Google. More recently, my dad found a fusion reactor design that makes ball lightning, I think it'd be fun to shoot that out of the rail gun.
There's also intermediate guns for power armor. An M240 designed to be fired one-handed like a pistol, and a modified M2HB. This would be modified to feed off a 20-30 round magazine put in a modified top cover. I didn't think a large magazine on your back with a chute going to the gun would be reasonable, and dealing with loading a belt with large, powered gauntlets I figured would be out of the question too. So, you'd double stack magazines to stick in like a normal gun.
I had a few other ideas, like the M2 above would have the mag release set up so that when the last round is chambered, the follower hits the switch to release the mag, and you load another. That way you don't have to worry about a holdback, or charging the gun. Thinking about a normal rifle, A story about Hammer's Slammers had a gun that had a paddle that blocked your sights when the magazine was empty. It might be useful, but a bit complicated, and would have to be designed so that if it breaks, the sights are still open, and it doesn't jam the mechanism.