Perhaps it would be a viable compromize to drop the "prefix+number" assumption
for entity IDs in general, but keep it for the kinds of entities we have right now.
This would mean that there would be a base class and/or interface called
something like NumericEntityId, which at least ItemID and PropertyId would
derive from resp. implement. That interface would then be required by storage
services that rely on the table structure described by Jeroen.