I have no idea if the engine allows this, and I doubt the modding tool even has the capabilities (atleast not yet) but:

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+Sieging weapons? Ballistas, trebuchets...

+Shields (which i am pretty sure they will be eventually added to the base game), and "Taketaba" (just google it, its pretty much a wooden mobile shield platform)

+Battering rams, siege towers...

+Being able to build pits and bridges (the ones that go up and down)

+The game doesnt have any caves... (Level -1 for buildings? idk, being able to build tunnels, pretty sure that will never happen but I am optimistic) So i would like to see some at one point :D

+Mounts: Horses?

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[Also if you need more people on your team we could exchange some ideas and tell you what I can offer as an addition to your ranks]

Ill continue Updating this once I come up with more ideas.

thats going to take a while, need to wate until the devs give us modders the code.

From my understanding seiging is going be apart of the game as is.. Though with the large group that this team is working with it may pay to talk with the developers to see if they may want collaborate and see what you can make. I am in the industry myself and we love it when the community builds useful mods that we can implement with their permission of course and credit given to them as well.
Also as for me I would love to see a more diverse economy to bring the game more alive such as a in town economy such as actually people moving in to building you make and buying goods and/or working for money instead of you having a massive group you only need to feed but pay as well. It'll add a bit of diversity to the game in my opinion.

Quick little idea, Throwable weapons, ie: knives spears kunia stars. May be possible to use mechanics form the mounted crossbow for this, which would save a little time. I mean u cant really have ninja's runnin around without thier ninja tools. Even just being able to throw your sword could come in handy, especially if its a rather heavy one.

Originally posted by SumYunGuy:

Quick little idea, Throwable weapons, ie: knives spears kunia stars. May be possible to use mechanics form the mounted crossbow for this, which would save a little time. I mean u cant really have ninja's runnin around without thier ninja tools. Even just being able to throw your sword could come in handy, especially if its a rather heavy one.

I like this idea alot ranged combat is a need