I've mentioned that I've been mulling the idea of a FATE/d20 hybrid. Today, I had a breakthrough. It was one of those things that seems obvious in retrospect...
In FATE combat, you can use a maneuver during an attack. Instead of inflicting stress (damage), a successful maneuver allows you to place a temporary aspect upon your target... or generate an effect of some sort. You can, for example, disarm or move an opponent with a maneuver.
In d20, there are a lot of special moves that you can do in combat. In Pathfinder, these are all called combat maneuvers. These include things like grappling, disarming, feinting, tripping, etc. d20 also has conditions. Some conditions are more or less extreme versions of each other (such as sickened and nauseated).
The breakthrough I had was to use FATE maneuvers not only to replace combat maneuvers, but to do it by imposing conditions. Essentially, a maneuver will allow a PC to place a condition on a target. An extraordinary success will allow a more extreme version of that condition to be placed on the target. Tagging an aspect will have the potential to do likewise. This also works well with stunts (or feats or whatever). An intimidation stunt/feat might make it easier for a PC to place a more extreme version of a fear-based condition on a target.
The problem is that the conditions aren't particularly balanced. I'm not a huge stickler for balance, but I certainly don't want there to be a clearly superior combat strategy in all situations.