Suggestions/ideas related to the news/dev diaries for the European Expansion scheduled for Q4 2019, which Paradox has described as focused on internal mechanics. Specifically, the last dev diary is focused on Estates.

TL;DR: Estates and Foreign Cores can and should be used to port/simulate key, celebrated CK2 features into EU4, namely vassal control and foreign influence of vassals.

1. All Estates should be able to revolt to become independent tags, like Cossacks do currently.
More on how this will happen below.

2. You should be able to influence Estates in foreign countries if you are powerful and wealthy enough.
This will have costs (ducats, spy network, prestige, relations, etc) and can either help or hurt the targeted foreign countries. At the most basic level, you should be able to incite foreign Estates to revolt. You probably should be able to help your ally to quell a revolt as well.

3. Foreign Cores can and should be linked to Estates.
This will put dead tags to good use. For example, dead tags with feudal governments should be linked to Nobles Estates in the countries who control the dead tags' cores: dead tags in 1444 like Kiev, Chernigov, and Smolensk can be used to simulate powerful magnates in Lithuania. Such dead tags' cores can be locked to Estates, or make certain Estates more unruly. Of course, dead tags disappears in time, simulating centralization.

4. Dead Foreign Cores should be able to pop up PEACEFULLY as Vassals when the central power is weak.
Rebel stacks should not be the only manifestation of unrest in your country. This will benefit from, but does not require, a rework of the Stability/Legitimacy system. Again using the Lithuania example. Your Stability/Legitimacy is low. Your army is weak. Poland abandoned you. Muscovy is lending its manpower for your Nobles Estate to revolt. Instead of waiting for Rebel stacks to pop-up, perhaps (with your decision to compromise) Kiev, Chernigov, and Smolensk will emerge as live tags again, as your vassals, with a temporarily increased (or decreased) liberty desire.

5. Vassals should be able to become independent, or switch to a different Overlord PEACEFULLY when the central power is weak.
Again, war should not be the only option. It is merely an extension of diplomacy, as a wise person once said.

6. There should be more ways to interact with vassals of foreign powers.

7. Estates can have separate manpower and ducats pools.
The regular tag AI can be used to spend them. This I think is in MEOIU already. It will make Estates seem more alive.