Would it be possible to keep the same formula that gives more flags before 500 and just carry it forward?
I don't think that anyone on SU will hit 100 flags (the current ceiling I think).
EDIT: See here for more details on the current system.
I don't think this is a good idea. What if a new user doesn't flag for a month? Then he has collected a ton of flags that he hasn't used, and therefore can flood the mods with flags that may or may not be valid. Flagging is a privilege that will be rewarded for good behavior. I feel it needs to stay that way.