To avoid blobs, I think we need better looting and higher walls. Or, maybe, we just need a culture change to convince people that looting is good and walls are high.
Looting is effective. The problem is that repairs are also very effective. Between Courtiers and Diplomats, I've seen regions go from 0 in every stat to 100 in every stat in two weeks. Eventually you get to a point where the enemy is repairing his regions as fast as you can damage them.
Although, I think repairs were made more effective because realms were beating each other up so badly that they couldn't actually fight each other anymore.
And walls are high enough already. Nobody sane attacks walls without twice the CS and a swarm of siege engines.
We're not going to solve a mechanical issue with a cultural shift.
My ideas:
1. Medieval armies usually take more casualties from disease than combat deaths. They don't know about germs and sanitation, just that folks start dying if you have too many of them camping together. The defending side usually avoid mustering until the last possible minute, when the enemy is well inside the defender's territory. (Partially due to slow communication methods, but also to avoid prematurely gathering a large camp of your vassals' men and have them start dying on you.) So, have troops start taking casualties to illness if you have, say, more troops in a region than that region's max population cap, or some percentage thereof. Make the penalty identical to the starvation penalty. Cities would have better sanitation and be able to house more troops, but if you have 3000 troops camped in a field, I believe it's fair to start punishing.
2. In addition to letting us TO regions adjacent to our own realm, let us TO regions that are adjacent to regions where we're already running TOs. Encourage the winners to spread out after a major battle and start TOing entire duchies, instead of one region at a time.
Realms will always blob up to fight. Bigger armies win battles. This is intuitive and fair. I do not believe it is possible to change this unless you alter the combat system so much that it's not even close to realistic anymore.
However, we can encourage realms to delay the blob effect for as long as possible before a major battle, and spread out as soon as the battle has been won.