Good thing but bad timing.
There are some anime series that were amazing but suffered from bad scheduling or were adapted from an incomplete source and had a different ending than the one in the manga or anime. Some suffered from production issues and the animation was a bit wonky, some had too many fillers that swayed from the main plot line, some just ended up forgotten. So people on Reddit talked about what old anime would become an instant banger if it was remade now to today's standards. And the answers were pretty interesting.
One of the most popular replies is Berserk, and that’s understandable: the story of this show is remarkable, the characters' development, the lore, the events that everyone goes through — it's one of the best manga series ever released, but the anime was created in that weird time when studios were learning how to use CGI properly, and in Berserk it was pretty bad. So bad, in fact, that some people got allergic to 3D in anime in general after witnessing that. So it would definitely benefit from modern technologies. CGI, however, was not his only problem: it also had a storyline that derived from the original and the sound design was questionable. And it ended right in the middle of the most intense moment of the series!
Another show that's been mentioned frequently is Gantz. There is, however, a question of censorship, as Japanese TV is much stricter in terms of nudity than manga. But the original show is pretty mild by modern standards, so probably they'll be able to even make it a bit closer to the manga in terms of nudity if they decide to reboot it. Then again, reboot can be released not on Japanese TV but on a streaming service like Netflix, and they have even less censorship. But that's not the only thing that would probably get a better treatment in a remake: the fights with the modern animation techniques will get more vivid and badass. But even the author himself doesn't think that it's possible. Wouldn't stop us from dreaming, though!
A lot of people mourn the anime adaptation of Tokyo Ghoul that in the latter part was so drastically different from the manga and completely butchered some characters. The manga has a unique artstyle that wasn't properly translated into animation, and the story got not just cut but straight up mangled, so this show truly deserves to have a FMA:B type of reboot, even if it's not that old.
Some also want a redone version of Reborn! (Katekyo Hitman Reborn!). We talked recently about how the popularity of the franchise dwindled, and the anime adaptation that didn't adapt some of the manga stories and changed some things (replaced the motorcycles with air bikes, for example) was one of the reasons for its demise. Aside from the questionable storytelling. Maybe not a remake, but a release of another season after a long hiatus would also work (we're looking at you, Bleach).
And another popular answer is Slam Dunk. The king of sports anime with an amazing story, it's often brushed aside because of its dated animation style. In the modern world, filled with sneaker culture and always starved for deep sports stories, it would definitely become a success.