Just fix the resolution and render the sprites again.
It's not that easy, increasing the resolution means increasing the view area which means changing the distance at which AI detects players. Also having exponentially more view area will fuck with the game balance due to the sheer number of more enemies that can now appear on screen/detect you. Also I doubt they even have the original assets to work with, the only thing they can really do in that scenario to improve the sprite-work is to run it through ESRGAN or some-such.
Diablo II still has the best itemization of any game I've ever played. It's amazing how clueless Blizzard is about the history of their own products, and why they were successful.
I think part of it is due to having 99 levels, and making said leveling experience take a long time. In D2 you're at about level 30 when you finish Normal, and even by the time you finish Hell you're probably not high level enough to equip the mid-80s Grandfathers/Azurewraths/Craniumbashers. Compared that to D3 where you hit 70 in a day and any non-70 Legendary is absolutely worthless (and for the top tier players, any non-set/non-Primal items).