Maybe it would be interesting to look at modelling the cabin atmosphere;

• partial pressures of O2, CO2, & N2
• bad effects of various imbalances; hypoxia, CO2 headaches & death, fire risk, ??
• leaks - Apollo leaked pounds of air per day, Soyuz is near hermetic
• EVAs vented cabin air => wasted O2 and N2

All good ideas guys. Keep the discussion going, how about we merge the ideas together. Will consider them for sure.
But something makes me think @nanomage that point 1 in your list, I've seen a mod do that in thepast somewhere. But can't recall what it was, and even if it is still up to date with KSP or not.

i am currently trying to make a plugin to do this. unfortunately i'm no developer so i'm painfully slow.
I'll clean the code up and upload to github since hooray now i'm not the only person to be interested in this.
This in my opinion is a much needed change because it lays groundwork for realistic scrubbers and makes air quality control necessary. It's totally fun, here just a small out-of-the-top-of-my-head list of things that will be achievable:

1. Pressurised vs unpressurised (unspaceworthy) cabins. Alternatively cabins differing by their leakage values, with bonanza being unspaceworthy by virtue of being super-leaky.
2. Scrub, freeze out, recycle CO2 in greenhouses/algal arrays,, or vent out the atmosphere and backfill from tanks.
3. Partial pressure of water vapour, condensation leading to increased malfunction chance.
4. Inefficient use of tanked oxygen and reduced level of astronaut comfort if the cabin air is unbreatheable.
5. further reduced astronaut comfort if the cabin air pressure is below Armstrong limit.
6. death if forced to breathe poisonous cabin gas, of course,
   not really:.farting astronauts producing methane and mercaptans to burn in the rocket engine

I believe it was BTSM that had that feature. I think RO does as well, but this is somewhat circuitous, as does not RO use TAC for this system?

So, I've been terribly glacial with it but i think i managed to start putting something together. I've put it up at https://github.com/nanomage/AirQuality. What it does at the moment is attach an atmosphere to a part and simulate people breathing it by modifying partial pressures of constituent gases.
@JPLRepo I think the best outcome i can envisage for it is if I can expand and polish it enough so that it can be integrated into TAC LS somehow (or any other Oxygen- and CO2-supporting LS mod). I guess the main part here would be to teach AQ to manipulate CRP resources.