This is really interesting discussion.
I will not try to repeat too much, but l am completely against any usage of lightbringers!
I will respect every decision of ET in term of them, but they are completely out of place in my opinion. I understand that Rivendell is treasury of knowledge (like Erebor is treasury of gold mainly) but presence of lightbringers as siege weapons, isn't my cup of tea... Simply leave force over elements for Maiar. I simply look at Elrond as "water bender" mainly because of his ring also Galadriel with her powers etc. Simply keeper of knowledge as those lightbringers shouldn't be able do do anything similar to elemenal bending... Some more realistic picture is needed in their case.
Concerning siege weapons as siege wepons, I agree with statement of Fredius. Simply I agree that Noldors were able to build and use ram, same with catapult, as with balista. There is no need to be so conservative about siege weapons in this term. It maybe look strange, but I think that siege of Angaband wasn't with bare hands or swords and bows. Same and more close situation to Rivendel and Noldors from Middle Earth is siege of Barad-dur. Ok there were Numenoreans, but again, I think elves had some siege weapons also.
All what is crucial is that model of that siege should be Noldorin style.
Noldorin, not elven in general, because take into consideration that they were warriors and smiths in first place and after that artists and singers. Some opposite to Vanyars and Telerin in that term.
Concerning current suggestions, I really like and support that Da Vinci catapult with elegant firing mechanism. It has really nice model.
Also that High Elven reaper thrower looks nice, it has classical mech of balista so it is real stuff.
What I find really nice is night elven glaive thrower which was mentioned by aminetude:
That siege weapon has already a nice design, and it could be adjusted in Noldorin style easily. It bolts are maybe more oriented as mass slaying stuff, but what I really liked in that model is hitting animation
. That kind of siege could be used as anti mass, anti gate and anti stucture except walls.
Some kind of ram is also something which should be reconsidered. From animal symols which could be used, maybe eagle head motive (we already have wolf,ram heads, can't remember is sige ram from rohan based on horse head look)? Just writing down some random thoughts.
Water, earth, fire, air and other "Avatar bending stuff" I would like to see only in hands of heroes/maiar. Also water/singing beding siege weapons sounds cool, but they are completely out of place in my opinion.
Who played game LotR: War in The North, there was that idea about sound, when two dwarven warriors are blowing some dwarven horn which produces sound of pitch frequency which corresponds to frequency of autooscillation of stone and brings whole Mount Gundabad down. It is cool, but not so eleven in general.
And that kind of machine should be very close to structure (or inside stucture) so it could work in general.