I don't often go through the review queue. But today I found a reasonably good edit there. Unfortunately, there had been another, better edit by another user with 2,000 rep privileges in the meantime. (They probably edited in parallel and submitted close to one another in time; the edits were both 6 minutes old when I checked time stamps.)
This means that the right thing to do is to reject the suggested edit in favor of the better and already live other edit. However, there was no way for me to say "This was a good edit, but unfortunately we don't need it any more". Every single suggested reason for rejecting an edit implies in one way or another that it was unnecessary when written, as apart from when reviewed. I think this is wrong.
My concrete suggestion is therefore that the reason-for-rejection poll includes something along the lines of "The suggested edit was good, but unfortunately it has since become superfluous."
Remarks: If this is something that should be on the general meta instead, please feel free to move it there. I hang almost exclusively on math.SE, so I don't know whether the reason list is specific to each site or not. Also, other suggestions for what the above quote could say are welcome.