I think this is quite a decent idea, would certainly fill up some time when you have nothing better to do in Cyrodiil. On the other hand, while some aspects are doable, the main aspect of this mod, is (at least to my knowledge) not. Siege towers might be possible, but it would be quite a lot of work to create, and script. Modelling, animating, and scripting all of these new elements would take a lot of time. Also 30 NPC's per team? 15 in total is Oblivions general limit, throw in a lot of other detailed models such as your catapults, and there could be some serious FPS issues. This mods not really possible, which is probably why you haven't seen it before. You could probably get a working version, but it would be on a much less grand scale, and not really what you expected.