I like the idea. I particularly like that it doesn't stack well with most other multiple-action features, such as Blackened Sky, since it demands a high chance of success. A few ways to spend it:
- A high prestidigitation archer could still try and stack it with Blackened Sky and Multi-Shot, if they used the Distracting Shot trick (AC) to make someone easier to hit at the beginning of the combo. (As a rules query - what would count as "success" for multi-shot? Hitting with any arrows, or all of them?) Further tricks for shots with a high chance of success (such as Eagle Eye from Bow Mastery, or the Practiced Shot trick) could also be added, but imho probably wouldn't be worth the proficiencies.
- Similarly, melee-combos intended to take down hard targets could start off well with Mix-up (Feint) and/or Toppling swing and/or Overpowering Strike and/or Forced Opening (not that you'd want a high error range with this mechanism), since these would increase the chance of success of other attacks. As I see you've done with the example you gave, now that I look at it.
- Any combo utilising the Practiced trick line would be good, since they're effectively auto-hits against many opponents.
- One thing this does stack well with visavis multiple attacks is the Cleave line. Any combo with a good chance of killing multiple standard characters (e.g. Blade Flurry + Shank from the Knife feat tree + whatever) could also generate further free attacks (and free moves, with Cleave Supremacy) rather handily.
Looking into Edge-generation, the only tricks I can see which specifically generate edge are tied to the Aggro, Lionheart, and Giant Slayer lines from the AC, although the effects of feats which grant edge from single or multiple successful actions such as Combo Supremacy, Qi Supremacy and Blocking Basics could also be fairly relevant (as Sletchman pointed out). However, most of the tricks that spend edge require large quantities, so you would generally need to chain multiple edge-generation tricks before using an edge-spending one. Not sure this is altogether abusable, although you could complement the Stout of Heart stance from the Lionheart chain rather handily with a Face of Death-[other edge gen trick]-Roar of the Pride combo, as long as you only take the final step when it's useful.