I personally think a more fresh approach would be desirable. In the end, it will certainly be able to do the various TresEd operations and a lot more. Still, I believe starting from "scratch" with the actual programming, as Rex is doin, will be preferable. There are some basic elements of TresEd which are not necessarily desirable and would be best to avoid before they are gotten to (such as, proper instance order management, not defaulting to the installation's \data directory, etc.). Rex has even talked about the possibility of "activating" physics in a pseudo-game mode

Of course, before that can even begin to work, we'll need the WTD format interpretation working, which was not even
fully accomplished with TresEd.

Heeeyyy. We have the source to TPDC, right? It seems to me the good ol'
GUIApp was used for working on dinosaurs, in some capacity, maybe. Basically seems to have been usable as an environment for working on any group of models including a single dino. Soooo... Maybe a TPM-opener function of the app could open a TPM as a scene, and include all the controls of TPDC, using its source (with full credit obviously).

Just an idea..
Oh and Rex, I know you wanted to make a new model format for use in Trespasser. I kinda like how it is now, with values as separate - among other reasons, non-TPM formats (such as 3DS, whenever it's used) still need values somehow..
Oh yeah. The insert-mesh function of TresEd is currently highly unrecommendable, because it has to take the mesh out of the GRF and there's no telling which geometry type it will use, which is an easy way to get a crash without realizing the cause. If you could implement this feature (with other simple models as well) and have it actually WORK, that'd be great. Since the idea is to combine all the apps, TresEd+ would be able to edit the GRF etc. properly to insert a new mesh (without getting it out of the level), and also of course the option to create a mesh should come with the question of what kind of object it is to be (desired class and/or bounding volume, etc.).
EDIT: Just a thought. Perhaps you could still make a secondary file format to be used, which combines EVERYTHING - model data, values, textures. Single file for the lot. This would expediate moving things between levels tremendously...
Aha! The file format can be known as .BWB