Union TAGs and sub-unions
One of the fields of interest where EU3 fell short of its own expectations was that of unifications. I have seen a unified Great Britain a few times, a few cases of Spain, and once France arose from the rubble left after the complete annihilation of France mostly at my hands. Ireland and the Netherlands appeared a few times, too, but without much fallout. However, I have never seen the AI form any other unions - Germany, Italy or the HRE for that matter.
Especially concerning the latter two, I'd put forth my idea of giving intermediate steps for the AI to facilitate the process. You know, that there be a series of safety nets - if the AI falls almost at the top, it doesn't have to start all over again.
So, I'd like if there were regional TAGs formable by proper-cultured states - Swabia, Greater Saxony, Westphalia, etc. - you get the idea, some 10-20 province TAGs so consolidate the progress, in great part by core removals - if a union (or sub-union) forms, the lower-level cores (or other mechanics granting releasability and independence revolts) should be removed.
Probably there should be a sub-union formation CB for states holding over half the required provinces (much as CK2's create/usurp mechanic works) and sub-unions should in turn recieve a free unification CB to create the Union TAG proper. This way bigger unions, such as Russia could form more often, I think.
To sum up, the AI should be given incentives to form the union tags - they exist for the exact reason to be formed. And this incentive should - from my point of view - come form granting a cheap CB for holding at least half the requirements, with intermediate levels if needed.
On a different approach, coalitions of the right states should sometimes decide to pull their ties ever closer, eventually forming a union TAG. I can envision multiple ways to implement it, of which the most all-encompassing would be the generalization of the HRE mechanics - allow mini-HREs to be formed in-game, and have the same mechanics as the real one, including dissolution and unification at the end.