I just started playing KSP a week or two ago. I tinkered with it a while back when it first came out, but with the introduction of Career mode and the non-spammable science experiments, it seemed more fun. I finally crash landed on Eve with a probe, and I'm almost through all the upgrade stages. I think I'm gonna build some more probe rovers before I commit any of my Kerbals to anything beyond the Kerbin system. Safety is Job 1. And because of that, I ended up thinking it'd be really rad if I could build a probe that uses balloons to get a measure of a planet's barometric readings, temperature ranges, and be able to map out the different biomes. It could even be used to probe Jool. They might even be useful for extended EVA travel on Eve, Duna, and Laythe. As far as ease of implementation, I think you could just change the models on Parachutes, and give the canopy a constant minimum lift at low barometric pressures. The difference being, though, that the balloon expands the higher it gets, where a parachute acts conversely.