I've not gotten properly started yet, but I've been rolling a couple of system tweaks around my head.
I think I've hit on how the change can work, I just need to run lots of test rolls and stuff (it's a ttrpg). The dice system is based in tiered difficulty (which tier is GM discretion), and it's heavy RP rather than crunchy rules.
So, I have the idea that rolling upwards of X number of successes beyond the max difficulty will potentially cause problems. So, if you're trying to punch through a wall, and you over-succeed the difficulty too much, maaaaaaaybe you just burst the city water pipes, that sort of thing - and it's limited only by the inventiveness (and cruelty) of the GM.
So that's where I'm at for now. After the end of this month it'll become my main project, so I'll get going properly!