Ballistae.

[The Wolf]

I do not think so - you would simply have to incorporate it in such a way that baricades still pay off.
Besides, I still see all 3 types of cades, even tho all engineers "should" only use heavy cades. The explanation is, ofcourse, that they are more expensive, and require a higher lvl to use. The same would have to be true for the balllistae.
It could also be a stationary defense system on some (or all) swadian maps, to be used by either engineers only, or everyone. We all know that swadia makes good use of crossbows, it would only make sense for them to employ anti-infantry siege weaponry such as the ballistae

I repeat: They should not be OP. And they would not have too - there are endless ways you could modify/nerf this feature and still have it be a great addition to the mod.

I'm starting to wonder how hard it would be to learn texturing and modelling :p even if the devs do not decide to use this thing, I would love to simply see it work in warband, somehow.

It's an interesting idea, as long as it's not "engineer only". Why would you want to be a medic if you could fire that thing? :p