I like the idea. And instead of using ability checks make it a skill check. I know a few rogues that would agree with this after getting repeatedly swept by a ranger then thumped. And it adds usefulness to a skill catagory that not many sink any points into.
Game mechanic idea...
Balance check vs. DC of trip attempt +10 might be an easy rule. The tripper rolls a d20, adds in Str, size, etc. The tripped needs to beat that roll and modifiers +10 to avoid being tripped. At lower levels the tripped would often fail, but if they sink more skill points into Balance they can roll to avoid the attempt. Will there be a break point in no one being able to trip the character? Sure, it would be similar to Tumble IMO, where anyone with 15 ranks can beat a DC 15.
But you have to get those ranks first, and it wouldn't be as easy. A fighter trips another fighter and has +8 for is trip check. He rolls a d20 and gets a 19. Added together it's a 27, then add another 10: DC = 37. Ouch. OTOH, a rogue trips another rogue with a +4, and rolls a 2, +10: DC=16. Not too bad at all if the target rogue has some Dex and ranks in Balance.
Players would have a whole new motivation to sink ranks into the skill. But it would add some complication to the opposed roll, perhaps slowing down your game a bit more. Eh, it's worth it IMO.
