The encounter power seems a bit odd and I'm not exactly sure what it is supposed to do. I take it the idea is you roll a d20 and then you have to use it for that power later on? I suppose you're trying to have some kind of element of risk to that - so if you use it on a daily then roll badly (like a 1) you basically shoot your daily in the foot if you needed it (and waste your action if you need to use it later on). I don't really like that mechanic at all. I can't see many allies picking that for any power they might actually want to use, especially if they know they won't be able to get a hugely advantageous situation to hit on very low numbers. I think the idea of having a roll for a power, then being able to choose if you use your fixed roll or whatever you got at the time would be interesting. Like a version of elven re-roll (or similar mechanics) that was limited to a specific power and which you rolled ahead of time.