Just a brief GURPS thought today.
I tend to find lots of stacking situation modifiers tricky. Players constantly want to "just make sure" that they get all of their bonuses. They want to deal with a long-term problem now with a focused investment of points. They want to eke out every bit they can to survive.
But making sure you've counted them all can take time, and still result in errors and edge cases - does Hard to Kill protect against a spell that might kill me? Does Hard to Subdue help me roll to wake up? Does my +3 for Dirty Fighting apply to this one opponent, this time?
On top of that, it can be point inefficient as well as unwieldly. The guy with HT 13, Fit, Hard to Kill 2, Hard to Subdue 2, and FP 13 has spent 16 points to get most of the effects of HT 16, but not all of them, and lost out on 0.75 worth of Basic Speed (15 points on its own) and 2 FP (6 more points) by not spending 14 more points. And has a lot more questions about what applies, when.
So what if you could only every buy one ability that provide situational bonuses? So you could get Fit and Hard to Kill, or Fit and Hard to Subdue, but not all three (and Fit only because it's not a situational bonus, it always applies.) You basically have to make a hard decision and get the one you really want.
You'd still get situational bonuses, but you wouldn't have a lot of them, and have to have so may iterations of your base stat + bonuses to keep track of.
I'd need to do more numbers and look at more cases, but this is an idea that might suit a game with high power and steady growth . . . while very high stats and skills might be annoying, are they more so than moderate stats and skills with lots and lots and lots of bonuses to comb through and decide where they apply? My experience says no. And it's better, in general, for the players - less questions, generally more effectiveness, and less edge cases where they thought something would apply and it does not. Simplified chargen and progression, too, is a helpful bonus.
So I think I need to put some more thought into what this would look like.