With the recent announcement of StarCraft: Brood War Remastered, it seems like Blizzard does have it in them to bring their older titles to life. While it's understood that StarCraft: Brood War Remastered is being done due to the game's continued presence in the South Korean market and its competitive scene, it does mean that they are willing to do a remaster as long as a game still has a significant player base in a market. That's where the question of remastering other games comes in.
Perhaps they could do the same with WarCraft III, which is still fairly popular to this day due to its map modifications. However, it's Diablo 2 that really makes the question worth answering. Despite being 17 years old at this point, it's still regarded as one of the finest action role-playing games ever, even when its gameplay features, Diablo 2 characters, and Diablo 2 items have been superseded and improved upon by many newer titles over the years.
Even now, it is still being played online by loyal fans. With that said, maybe a Diablo 2 Remastered wouldn't be out of place, despite the presence of Diablo 3. But the deal with StarCraft: Brood War Remastered is that StarCraft 2 wasn't able to achieve the level of success that Brood War had, especially in the South Korean market, which is why they decided to update Brood War and revive that scene.
Meanwhile, despite the rocky road during its first few years, Diablo 3 has been doing well for itself and is certainly far from being a flop. Therefore, Blizzard may not have that much of a reason to do a remaster of Diablo 2 other than demand from the fans. But no matter how overwhelming that demand may be, Blizzard won't just budge and give in to it unless they have something else that compels them to put time and resources into actually remastering Diablo 2.
They have shown their willingness to somewhat continue development for Diablo 2 with the release of the latest patch a while back. They never really had to do that, but they did since they are aware of just how much fanfare their old work still has. While Diablo 3 is still being played by numerous fans worldwide, it can be said that it's not a lot like the popularity that Diablo 2 had back in the day, so there's still hope for fans somehow.
If ever a Diablo 2 Remastered comes to be, it'll certainly reinvigorate interest for the old game for a good while. Perhaps the online multiplayer can be tweaked a bit so that it can remain interesting in this day and age amid all the other action role-playing games now available on the market. If there is ever a game development company capable of accomplishing such a task, it is Blizzard Entertainment.
While the remastering of StarCraft is mostly a graphical overhaul while preserving virtually all of the gameplay since it's the competitive aspect of that game that is most important, Diablo 2 may need a bit more work. Things like quality-of-life improvements and some gameplay tweaks may be necessary.
Overall, Diablo 2 Remastered may indeed be a good idea for Blizzard to execute as there's still a lot of love for the old game and it could use an overhaul for it to be introduced to a whole new generation of fans.