It isn’t much to go on, but a job listing from Blizzard Entertainment suggests that a remaster of Diablo II could very well be in the works.
It was only in March that Blizzard announced the development of Starcraft: Remastered in which they’d completely overhaul the graphics assets to support 4K Ultra HD video cards and monitors.
So, for them to potentially be doing the same for Diablo II isn’t unlikely.
The wording in the ad removes any doubt for me that the plan is to indeed overhaul classics like Diablo II and, as the ad alludes to, even Warcraft III. “We’re restoring them to glory” is some strong and emotive language that speaks volumes.