I really like the border war concept, but I'm a little confused by the last DD. I feel like it lacks balance, and also interest. Let me explain. The goal of this idea is to make powerful empires more fragile, even if they are led by a player.
Imagine two scenarios:
- A conqueror is rampaging nearby and swallowing lands that you have claims on, so you are left with three options: murder them, fight head-on, or roll over and die. - You restored Rome, again. And didn’t choose the hard option, again. Now you are out of challenges and can either get to tediously conquer the world or start a new Rome restoration run.
If I interpret the first image correctly, as a player I can support my vassal at any time via the "Protect vassal" interaction. This does not make a player-led Empire more fragile, because as a player I can always support my vassals if necessary. On the contrary, it risks only weakening AI Empires, and therefore reducing the challenge for the player who controls a larger realm.
The second concern is the potential casi bellorum you can use. The second picture shows a king attacking for an entire duchy within the HRE. It's way too big for the Emperor to not immediately intervene. Especially since it is probably possible to invade an entire kingdom as well. Border wars should be restricted to counties (or at most duchies for hegemonies), and probably to attackers who are only counts or dukes (or kings for hegemonies). I really don't see in what world the King of Poland can seek to invade the whole of Bohemia without the Emperor seeking to defend it.