I'm not so sure on destructible walls, those on screens seems pretty small, "city-size" walls would probably require double the amount of pre-made scene props. But, yeah, TaleWorlds are the big boys with big toys here, not random modders, so let's hop above making them, let's hop above implementing them, let's hop above implementing AI that can handle breaches...
...we're still sitting on the fact, that it's something that can randomly make dozen of your soldiers go *poof*. Which is more or less one tenth of your army. Or can take you down, because you actually wanted to do something during siege, instead of sit through a whole thing in a safe-spot.
As I've said in other thread, siege weaponry killing 10 of ~100 men strong unit in TW? Welp, life is brutal.
Siege weaponry randomly killing 10 elites out of your ~100 men strong party in M&B? Guaranteed rage inducer.
Now, I wouldn't have anything against elaborating the "implied" work of catapults and trebuchets that we've got now, when siege towers aim for parts of walls that were already taken down. Maybe being able to "create" more such attack points with higher engineer level, or maybe even being able to create breach and assault such choke-point. But I can't see actively throwing projectiles around, mowing through yours and enemies' ranks. That's worse than "head-seeking" javelins.