I've posted this before:
As far as I can tell sieges have not been reworked since the very first Europa Universalis and, frankly, they suck.
They are terrible for three reasons.
1) They do not reflect historical siege warfare. EU4 is a game that doesn't really try to reflect history, so this reason is the least important of the three. Paradox devs are never going to make fighting in winter as difficult as it was historically, and they'll never make it so that taking an intact force as difficult as it actually was (indeed, the last time this topic came up we had difficulty finding a single incident of a prepared, undamaged fortification falling to an assault. Everytime a fortress fell to assault it was either stormed in a surprise attack, taken with the aid of treachery, poorly maintained, or damaged by siege craft first).
2) They kill balance. In CK2, larger armies beat smaller armies 99 out of a 100 times unless the smaller army was very unusual. Smaller nations and alliances have almost no hope of beating larger nations. Why even bother fighting most wars when the outcome is known from the start? Making a more interactive form of siege warfare would finally give smaller nations the tools they need to protect themselves from larger attackers.
3) They are bad gameplay mechanics. This is by far the most important reason. They aren't fun. They aren't interesting. They aren't exciting. I've never seen anyone argue that siege warfare in Paradox games is something they enjoy; most supporters say that they simply tolerate it.
Making siegecraft a bigger part of warfare would give more choices to the player. The men inside the castle should be somehow connected to a nation's manpower, so that every man in the castle is a man who can neither sow the fields nor invade the enemy (and, likewise, every man killed in battle should harm a nation's economy as well as it's ability to defend itself). Sieges should be more interactive, where both defender and attacker should make moves and counter-moves in order to try and outflank and defeat their opponent. Diplomacy and treachery and logistics and strategy and characters involved on both sides should all come into play. And, above all, sieges should be risky. The besiegers should lose more often than they win when besieging major, built up fortifications during the first couple of centuries.
As the centuries pass capturing fortified areas should get easier, though holding on to those areas should become harder due to forces such as nationalism.
This might sound like an impossibility, but since the current system is awful, making a better one probably won't be too difficult. Indeed, Magna Mundi (the game that ended up crashing and burning, not the mod) had what looked to be an interesting yet still automated take on siege warfare, a take that could be further developed into a better system than the terrible one we have now.