[Thrawn2]

Nice.
Some ideas: include some natives and forwards, so other plugins could use this as a backend. You would need natives like GetClientLevel(iClient);, GetClientXP(iClient); and a single forward OnClientLevelUp(iClient, iLevel);
With these one could easily extend the functionality of your plugin, allowing real benefits from leveling up.
If you add SetClientLevel(iClient, iLevel); and SetClientXP(iClient, iXP); one could also create a (sub)plugin which saves the xp levels.
Small additions, large effects =D
edit: oh, i just saw you had that already on your to-do list.