Diablo 3 has evolved over the years into another big Blizzard success, but it wasn't universally loved when it was new. One of the biggest criticisms it faced was over the art direction, which many fans felt was too bright and colorful for the series, when compared to the oppressive darkness of the first two games.
It wasn't originally envisioned that way, however. "New" screens recently collected at PureDiablo suggest that in the early stages of development, Diablo 3 actually looked much more akin to Diablo 2 than it ultimately ended up being. The images were originally posted several months ago by Oscar Cuesta, a former artist at original Diablo studio Blizzard North, which was shuttered in 2005, but they went unnoticed until they ended up on the Diablo fan site.
Cuesta said the models "were pretty low polygon and we were only using base color maps," and they're obviously incomplete, but even so it's clear that the initial vision for Diablo 3 was a much more direct continuation of its predecessor: Grim and Gothic as hell (literally), but relatively restrained and muted. I'm not so sure it would've been better, strictly speaking, but it definitely would've been a better fit for expectations.
Despite the fact that Diablo 3 is still going strong, Blizzard indicated last year when Diablo 4 was announced that it will swing hard back to the gritty darkness that we remember from the first two games. Whether or not they're actually as dark as we remember them is a matter for debate (a topic that Tyler and James dove into last year), but last year's intro cinematic is definitely all-in on the unpleasantness.
Diablo 4 currently doesn't have a release date, but director Luis Barriga warned when it was announced at BlizzCon 2019 that it's "not coming out soon. Not even 'Blizzard soon'."