Customizable stable regions, separate from the de jure maps. Currently, if we want to limit an event to the geographic region of, say, India, we need to use the de jure maps to limit to land within the de jure Indian duchies, kingdoms or empires. De jure drift has presented an issue for this in the past, but it would at least usually be nearby titles that land would be drifting between. However, with the new kingdom creation feature, we could end up with titles in the correct area, but outside of any of the de jure titles we can realistically code for. (Short of specifying each county individually in the events.)
Having a system where we can designate land as being in India, or Iberia, or the Middle-East, or central Europe, and use that for triggers, or as input into something like capital_scope would go a long way. At least, I think so.
Edit:
Being able to have it so we could use it to send everyone in a region an event would be great, as would being able to use it to limit the potential for events to trigger to people within a certain region.