...Part One...
...Waves...
Here's how I think siege could be made a lot more fun, first of all, make people spawn in waves, make those waves by 30 seconds for attackers, and 30 seconds for defenders, make balance work so attackers get twice as many people as defenders... now increase rounds to 30 minutes each, make each map only have one round in total, increase upkeep % appropriately...
...Part Two...
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...Fixing CS shit...
Make catapults work against walls, enable ladders, weapon racks, and forward bases. Make CS sites break in two hits by most weapons, but allow for players to spawn a few barricades with varying health and shapes, make them each require a CS only, add a new kind of weapon rack, an indestructable uncreatable one, which can only be placed from the scene editor, enforce that all siege maps must have at least one of these for attackers and defenders.
BTW: ADD A BALLISTA!
Change how ladders work: First of all most of them need to take three slots, with the largest one taking an entire four, and the smallest one only taking 2. Do so that they require at least one person at the bottom of the ladder, not standing on it, activating it by holding F, for every two guys on the ladder, if this is not the case then the ladder will break after three seconds. Now, also do so that if there are more people at the upper side of the ladder pressing F on it, than on the bottom side the ladder will fall, spawning an equipable "damaged X (X=ladder type) ladder on the ground, a damaged ladder can be put up again, but got half HP, and if broken breaks for good.
...Part Three...
...Maps...
With these changes you could expect engineers to become much more common, as in, common enough for devs and map makers to balance their maps expecting at least five people willing to use CS shit and/or ladders, and due to how the spawn time, map time, and spawn wave changes would affect the game mode you'll need new much larger maps, with balance around logical attacking and defensive strategies instead of random chokepoints the map maker wants to add, so this would require pretty much all current maps to be removed.
...Part Four...
ADD BOILING OIL YOU CAN POUR ON YOUR ENEMIES FROM THE WALLS!!!