Its important to make the world feel larger and more epic like it was in D1 with darkforest, more cities of diferent races, etc. What is realy lacking in D2 (on contrary to D1) is the race and cultural diversity.
What realy captivates people in RPGs (from my experience talking to people) is that diversity is a key element. People like visiting a dwarf city, or an elven forest, or a (insert Race X here) environment. It makes the gameplay diferent and its clearly separated in our mind as "an extra spot" and makes the game feel more epic, larger and more "alive".
This is, imho, the biggest thing it could be done for D2 expantion or D3.
