Even though different developers are doing the different versions, all assets will be shared. I highly doubt the next gen version will have a bigger map, more varied terrain/textures, different buildings, bigger cities, more destructible objects, mud physics for rally racing.
What it will have is better effects and lighting, stable frame rate, longer draw distance, higher resolution, more detailed driving model, higher poly count car models, higher res textures. (or at least high detail range will be further)
Filling up the available disk space and ram with hugely varied sprawling landscapes would have been preferable. That could still happen, TDU unlimited somehow got crammed on the ps2 as well, and still felt plenty 'next gen' at the time.