I suggest adding the ability to LOWER any skill one wants at-will (or a range of appropriate skills---I haven't much considered if this would break anything). It might, of course, let people determine critter to-hit bonuses with precision if applied across the board. But here's an argument from preserving realism: If I'm an expert fighter, I should be able to swing like a fool if I like.
Realism is not what prompted this post, however. I'm currently working on perfecting body-pulling by generating noise. As it turns out, I'm just too sneaky to make noise. (I haven't tried all possible tactics yet, and I'll continue to try new methods of generating noise, but it's quite silly that I can't easily make as much noise as I want whenever I want.) Nor should this be difficult to do, and it's currently a royal pain. If I'm an expert at Moving Silently, I should be able to exert fine-grained control over how silently I move. It would also be nice if lowering one's Hide/MS ability allowed one to move faster while sneaking.