We thought of something similar: The global list of event categories should be rather short in order to keep it simple creating an event. Having this approach on categories it is impossible to display / group / filter specifically. Example:

Our IT wants to use the events for planned downtimes and maintenance. Instead of creating a category for all the different systems that could be affected we want to provide them a category "IT Systems Downtime". If there was an additional layer for categorising (e.g. labels) that can be used for this. We would prefer a preset on space level though :)