Balloons!

[jebster]

I think balloons would be pretty neat! They could be like the opposite of parachutes. Mount them somewhere on your ship and then add a wet container of some kerbal version of helium. When the stage is activated, the balloons fill up and drain the wet containers. They would have unique physics and functionalities, such as not working in vacuum but being especially effective in denser atmospheres like that of Eve's. I didn't see this on the already suggested list so I thought I'd share. Thanks for reading!

[Orbiter]

In a very simplified version of this, balloons exist in the form of Hooligan Labs airship parts. They include multiple envelopes that can fill with a variable amount of lighter than air gas. Your concept of limited helium is interesting, however, may not be necessary. While helium balloons do leak over time, you would not need a large amount of spare helium for an average flight (the player's attention span). As for this becoming part of stock, that would be an interesting touch for Eve ascents and other planetary bodies with atmospheres. A modified Hooligan Labs airship envelope would be quite entertaining though.

[Ekscalybur]

Balloons would be a viable 'space elevator' design, getting around that whole cable becoming heavier than its ability to carry headache. Four platforms each holding large fillable balloons would allow them to lift variable weight payloads to just shy of the edge of the atmosphere.

[deckard009]

This would be great to put a probe in Jool atmosphere, without risks of aerobraking or failing. You can put a permanent station wiht Kerbonauts in Jool as well using balloons

[ixolate industries]

Try and put a balloon on a probe for jool and it should really plummet like a rock if you assume jool has a similar composition to Jupiter Ie hydrogen your balloon would have no buoyancy at all.

But who really knows about jool

For Kerbin and Duna though it would be great.