Human powered submarines:
Rather conventional seeming solutions, but nice anyway.
This is a more out there attempt. A flapping submarine, like a ray. It doesn’t seem to go very much anywhere when one compares to the propeller ones:
Festo has built an Air Ray, a remote controlled helium balloon with air muscles for flapping its wings, and it works nicely:
One possibility for future aircraft is incorporating flexibility again for morphing to increase performance.
Lisa Akoya already has a stretchy skin that comes out with the flaps between them and the main wing to give more wing area that enables low speed landings so the craft can stay in the ultralight category:
Reusable launch vehicles as well very likely need changing geometry to land comfortably.