Add name changes.
Increase game size to 12-16 players.
Make PvP zones specific for dueling.
Better Terror Zones / more consistent good ones.
Possibly some item additions, updates.
Otherwise no dramatic changes. D2R is in a great spot in my opinion. Id hate it turn into d3/d4.
We love you, Blizz!
Then you should properly define what that means.
IMO any mechanic that currently works as-is in D2 is fair game. So, take the countess rune drop special and add it to the other super unique enemies. Maybe not all, but at least Summoner and Nihlathak. Give key farming some extra oomph, and give those of us who choose to farm (more efficiently) for our first high runes some variety other than tower runs.