Suggestion: Wet Space Configuration options.

[MadMerlyn]

Here's the idea in a nutshell, converting empty tanks in orbit or in the VAB to use as hab space. Could require material kits and/or colony supplies to perform the conversion after launching, and have fund cost associated with doing it in VAB. This would allow you to build aerodynamic Ascent vehicles for places like Duna or Eve using mk3 fuselages without packing hitchhikers inside a cargobay or something to that effect.

Could also have a wet lab configuration, but have it be a less effective version than the stock MPL etc.

As inspiration, here's an ascent vehicle I've modeled after the Lockheed Martin Mars proposal, but with defaults it has limited hab time: https://i.imgur.com/EafdIH1.png