Howsabout you all stop worrying and just wait for the results? Onra knows what he's doing and most certainly will NOT produce something which replaces the Cyrodiil parts of the existing heightmap. Doing so would just be foolish and Onra is no fool. Such a mod would probably be incompatible with Oblivion.esm!
This could be stupid question, but why don't you simply copy Iliana's landscape records to your Tamriel plugin? That way they would be the same and there wouldn't be conflict. Am I right?
That would be a working solution to the problem. It wouldn't even require the texture info, just the landscape normals and height data.
No it wouldn't (at least not exactly), not unless Onra made Tamriel Heightmaps dependant on Elsweyr. Because new cells conflict in the complete opposite to how everything else conflicts (normally last-loaded wins and overrides any conflict before it, with new worldspace cells in an existing worldspace, earliest-loaded wins and overrides [underrides?] everything loading after - and if a cell gets overridden, all non-persistent objects in that cell will disappear) if Tamriel Heightmaps were to simply contain a copy of the cell records and heightmap from Elsweyr DoA and loaded first, they'd override Elsweyr as the Form IDs would still be unique so all non-persistent objects would be missing - then a patch would have to relocate literally every single non-persistent placed object from Elsweyr DoA into the Tamriel Heightmaps cells. If Tamriel Heightmaps was loaded last, Elsweyr would override Tamriel Heightmaps, and a patch would still be needed along the borders of Elsweyr's cells. Actually in my ignorant opinion, this sounds like a reasonable move - have Tamriel Heightmaps load after Elsweyr, so that those not using Elsweyr would simply have the Tamriel Heightmaps cells, and those using Elsweyr would have the DoA cells and a patch for the DoA cells border. This patch would be smaller than if the two heightmap mods were swapped around.
Onra will already have considered the most reasonable way to do things, and will make clear how it all works once it's ready to work.