Since I can’t look at a system without wanting to hack it, here’s how I would hack BRP:
Replace d100 with d20. That means Characteristic Rolls would be straight d20 vs. Characteristic, instead of d100 vs. Char x 5. You’d have 1/5 as many points to divide among your skills (50 instead of 250, e.g.).
Rolls would all be “blackjack style”: roll as high as possible without going over (BRP used to work that way, at least for opposed rolls, and it’s still listed as an optional rule). Hitting your number exactly would be a crit, a 1 would be a fumble, “Special” would be figured on getting close to your number (within X, where X ranges from 0 to 4 depending on your skill).
Why would I do this? Well, basically because I don’t really believe variations of less than 5% add anything to the game, but the math with all the double-digits and multiplication and division is fricking annoying. d100 is really intuitive: a 51 is 51% chance… but that’s its only advantage. My experience is that players just don’t roll enough to discern the difference between 49%, 50% and 51%. That means deciding whether to put 49, 50, or 51 points into a skill at character creation is a waste of effort, and tracking whether experience raises that by 1, 2, 3, etc is a further waste. Call it 10 on a scale of 20 and be done with it.
Meanwhile rolling low is less intuitive than rolling high, but if you don’t use roll low w/d100 then checking for crit or special gets really ugly: you basically have to look at a cheat-sheet next to your skill every time. d20 lets you say a 5% chance is the same as rolling your to-hit number exactly; that’s a little more generous than straight BRP, but it’s super-easy.