When Diablo 2 came out back in 2000, it was a revolutionary game as it built upon the template set by the first Diablo and all the other hack-and-slash action role-playing games that were made during that time, then made them all look primitive. Diablo 2 was jam-packed with features and its world felt so big that it truly was a game-changer in the genre and all of video gaming. But it's now 17 years later, and much has changed since then, yet Diablo 2 is still played to this day.
The question now is what would happen if Diablo 2 were made today, disregarding the fact that the action role-playing game genre would not be what it is now if Diablo 2 were not released at all back in 2000. What if the game were made with today's technology and game design knowledge? That's a question a game designer familiar with action role-playing games would be able to answer.
Most who are in-the-know would answer, "Much like Path of Exile." That game made by a bunch of dreamers from New Zealand became the new go-to action role-playing game, greatly inspired by Diablo 2. Perhaps they are right in that a lot of what Path of Exile has now established would be what Diablo 2 can be if it were made in this day and age. But a lot of the same things that makes Path of Exile great are directly a response to the flaws that were eventually revealed in Diablo 2.
For instance, Path of Exile's currency items are a direct response to Diablo 2's economy transitioning from gold to runes as medium of trade due to hyperinflation. Come to think of it, Diablo 3 had a problem with its economy as well due to the Auction House, which was removed in 2014, which actually made that game better. If Blizzard knows what they know now with in-game economy, perhaps they would make use of some sort of item trading system in Diablo 2 instead of just gold, perhaps make them coexist with gold being a secondary currency used for trading with NPC vendors.
Then there is Path of Exile's most prominent trait—the skill tree. Perhaps Blizzard would not do something as extreme as what Path of Exile has, which is a veritable skill forest instead of just a tree. Judging from the skill system in Diablo 3, Blizzard has been trying to do something new with it, but it can still use more permanence. Perhaps they can take the permanence of Diablo 2's skill tree, the sophistication of Path of Exile's skill tree and skill gems, and Diablo 3's use of hotkeys and different versions of the same skill, and combine them all into one system that works.
Other than that, perhaps a few tweaks with Diablo 2 items, removing less-used systems and item types, quality-of-life improvements, and other things to have a Diablo 2 that is best in this day and age. It can be fun to speculate on what-if scenarios, especially with talks about a Diablo 2 Remastered going as strong as ever in 2017.