"Upon the south side of the lawn there stood the mightiest of all trees; its great smooth bole gleamed like grey silk, and up it towered, until its first branches, far above, opened their huge limbs under shadowy clouds of leaves" - The Fellowship of the Ring, "The Mirror of Galadriel"
I would like to suggest placing a Fangorn-sized Mallorn tree right in the middle of the hill of Caras Galadhon. This makes it far easier for players to construct Celeborn's House, because 'normal' giant mallorn trees are still too small for a proper tree-palace.
NOTE: this is not a fixed structure, just a tree. In my opinion, it is equal to the hills/mountains that are already in the mod (Caras Galadhon, Edoras, Mt Doom etc etc).