I think something that needs to be added is, rather than just more tags, is the ability to change a nation's name/flag/color without changing the tag, either by decision or by player action (or specific AI coding). It's proven that the three are distinct (see: colonial nations, client states). So rather than creating a separate United Kingdom tag, create a decision that will change the name and flag while keeping the GBR tag. This will also solve the problems of multiple similar countries existing solely because they have different tags (like France and Rev. France, or having Naples, Sicily, and Two Sicilies all coexisting). Since a lot of the proposed formable nations are not culture unions, there's nothing in particular that needs to be added to the tag.
Though, for that matter, you could just add the culture union to a tag while just changing the name and consolidate the number of tags. Scotland forms Great Britain? Instead of changing to a GBR tag intended for England, it stays SCO but changes its name (and color slightly, similar to ENG -> GBR) and gains the British culture union status.
EDIT: Relatedly, add dynastic tags similar to CKII (for certain Muslim states/China/etc) that automatically change name if dynasty does.
