Actually there are other mods that use assets from Baldur's gate etc. And there are mods that have a total menu overhaul and high-res as a standard setting like Le Royaume des Ombres. I expect Sigma will have a similar 'new feel' like that last mod I mentioned when it comes to a redesigned skin. Also, mods like Reign of Shadow have little skill icons when you're cursed and buffed, so a lot is possible.
But full 3D HD models.... I guess the engine won't allow that
Full 3D HD Models might be impossible, but projecting that 3D model onto 2D Sprites maintaining the same layers could be possible.
What others ask isn't really a GFX improvement to the 2D plane of the game, it's changing the "Rendering" from a flat 2D plane to a full 3D one, that won't be possible without replicating the game core mechanics in a different engine that allows rendering in full 3D, sadly, the game itself relies on the current rendering scheme for most of the mechanics and would be almost impossible to "Convert" what currently is there, would be better to just Redo the entire thing in a new engine with new assets (becomes a separate game by then) and would take far too long for any meaningful improvement (nobody really wants it, we're just fantesizing about what it would be like).
However ... Since it is a 2D Top-Down Isometric view ... Maybe making only the character/monster models 3D could be done as there's plenty of games that use the 2.5D scheme (2.5 actually just stands for midway between 2D and 3D where either the background OR The entities are 2D or 3D, not both)
On some of those, the background is 2D (Ground, Buildings, random doodads) But made in high resolution textures filling in as much detail as they can, and the Interactables (NPCs, Objects) are 3D, but are rendered almost bidimensional (projected onto a flat 2D plane then rendered on top of the background or behind it in some cases like trees or fence/building with the occasional fading of the background to see your character) This might actually be possible, but would require extensive work to Convert all 3D models into 2D sprites (those games don't need to do this as they only need to project the 3D model onto a 2D texture) so even this, might not be worth it just to improve visuals.
TL:DR - There's far too much "Investment" Required for such an idea to be implemented, and you may not like what you get at the end.