As things stand now the game is to avoid a vassal rebelling in the first place by playing the relations game. I guess I'm ok with this. But fighting rebelling vassals is not part of the challenge - if a vassal you're annexing rebels you've already lost. It can be an OPM with 0 troops and you can send in your megastack to assault instantly and end the war in a month, but you have to restart the annexation timer. That's about as fun as if they introduced random events that regress your tech by one level or cancel the last alliance you formed and wipe your improved relations with that country.
If vassals could rebel at any time it would be necessary to freeze the annexation timer, much like coring timers are frozen if you go to war with someone who holds a core in that province. Maybe you even need warscore to demand that the annexation continues or something like that. And maybe if the vassal gets enough warscore they can cancel the annexation, if they get more warscore they can declare independence. Or whatever implementation works, but something would be necessary.
I guess I'm opposed to any quick change that makes vassals more aggressive, because there WILL be unintended consequences and then it'll get "tweaked" every single patch. But I would gladly welcome a well planned revamp of the system that makes them more aggressive and a greater challenge overall.
As a side note, has anyone experienced whether or not royal marriages help prevent rebelling? Daimyos could always rebel since vanilla because they're daimyos, and I always solved that by marrying them. I assume the same works for regular vassals now and I have not had any problems, but I haven't played enough to know for sure. What I have noticed in 1.5 however, is that in both games I have played to the point of becoming shogunate/becoming independent daimyo, my daimyo vassals are ALWAYS aggressive attitude, even after sitting at +200 relations and when I hold no land with their cores. And that stops me from marrying them even though I used to be able to (or maybe as shogunate I still can, I forget). The only options are to waste diplomat time by keeping them always at +200 or to risk it.