I have completed a pretty substantial change to the way people use Powers in combat in my beta version of Heroes By Trade. My old system worked by letting people use low ranking Powers at will but if they wanted to use Powers of a rank higher than their Vigour things got tricky. The Power would still work but using it would Fatigue them and each turn after that they would have to roll to see if they recovered from Fatigue and could use another Power or not. The higher the rank of the Power you had used, the harder it was to recover from Fatigue. The system was perfectly fine in terms of balance because players had the choice between using weaker effects consistently or stronger effects occasionally.
Balance isn't what it is all about though. What the game came down to was people would roll 1d6 on their turn to see if they were going to do something cool or something boring. Roll high and you get to select an action from a big list of interesting Powers, roll low and you just make a Basic Attack. What it came down to is that people don't really enjoy rolling to see if they get to have fun this turn.
"Roll to have fun!"
"I failed the roll."
"Haha, no fun for you!"
Rolling to see if you hit is fine. Rolling to see if you have fun, not so fine.
My new implementation changes this somewhat. You can still use any Rank of Power you like up to your Vigour. If you want to use a Power of greater Rank then you have to spend resources to make up the difference. If I have a Vigour of 8 and want to use a Rank 12 Power I need to spend 4 Focus. The key to the bit is that Focus is both my Hit Points and my attack resource. I can use outrageous Powers that do crazy things to the enemies but I will use up my Focus and if I get pounded on I will have nothing left to defend myself with. This means that no matter what I did last round I can always do something interesting this round and I never end up unable to do anything fun for most of the fight through poor rolling.
This means that people will be allowed to burn themselves out doing incredible things if they want or play it safe if they want and they can switch it up from round to round or fight to fight. It also means that if they players ever feel like a fight is just taking forever and they aren't worried about losing they can start burning away their Focus like crazy to end things quickly and I like that as a safety valve. Plus we avoid having one mandatory die roll at the start of each player's turn (which they constantly forgot about!) and we also don't have to remember which Power was used last.
The best part about it I think is that my design space for writing Powers got huge. Now I can write things that have ludicrous effects and let people burn away all of their defences casting them if they are in the mood for that. The old limitations on Power effects were fairly stringent because they only went up to Rank 14 but now I can write things at Rank 28 if I am in the mood and that lets me really do some wacky stuff. I am glad to provide players the opportunity to go all Tellah casting Meteo ("So be it! Let me life fuel the spell that ends his!") and blow the enemies up good in the name of an epic story.