Agree, if you add highres textures to old games you quickly reach a point where it just feels less consistent than before, because everything is pretty low tech and then you have these out of place photorealistic textures. It might just be me, considering how popular high res texture packs for Minecraft are. I always felt the stock ones or at least those that tried to mimic the 8 bit style looked best.

So yes, upgrading models and even the level geometry would be very cool IMO.

Depends how well it’s done. Check out https://www.moddb.com/mods/quake-15 for Quake (I), for example.