I'm not sure when/if I might actually get around to something like this, so I'll post it here.
I think girls should have limitations to the maximum they can raise a skill or stat to. Having all girls capable of getting 100 in pretty much everything is a bit boring. I think it would be better to have different limits for each girl, so some girls will be better at certain jobs than others.
The way to get this limit would be two ways. First off, the limit could be specified in the definition for each girl (in the girlsx/rgirlsx files). If not specified, the limit would be determined some other way. Perhaps something like 50 plus the starting value of that skill/stat? There's probably a better way to determine it, that's just off the top of my head.
One consideration would be whether skills and stats added by traits or items would be affected by the limit, or instead increase the limit by that much.
I think along with that it would be nice to have the increases in stats and skills from training or from doing work slow down as the value increases, with them gaining quickly at low levels but more slowly the closer it gets to the maximum value. In other words, similar to how gang training works now. Specified limits over 100 (in the girl definition) could probably even be used to make the girl gain quickly all the way up to 100 (not higher than that, though).
Of course, a shortcoming would be that you can't currently specify what types of sex acts each individual can do. That would probably be a good follow-up to be able to have girls focus on the kind of whoring they're best at.