There are currently 4 Custom Options available (go to the tab named Custom Options):
You can decide to display the zone and coordinates of the rare or remove them.
You can decide how long you want the currently spawned rare notification to stay for.
You can decide how many rares you want to see in advance.
You can decide which rares the WeakAura will track by unticking the box next to their name.
Leave your suggestions in the comments for any ideas if you have any!