I know someone already mentioned this series, but Fallout.
I really didn't like where the series went after Fallout 4 and having it go back to its roots as a gritty top-down RPG would definitely bring a lot of fans back. What Fallout 4 lacked was the feeling of being in a dark, twisted, dark world. Fallout 4 was too bright and wasn't very good at making a tense atmosphere like the first 3 games plus NV. It's an odd complaint, but I didn't like how umm..civilized everything was in 4? I much prefer the more crazier and unpredictable cities of Fallout 1-3. These older games had cities with more interesting characters and buildings. For example, in Fallout 1 there is a town full of ghouls that lived there after the bombs dropped because no one else accepted them. Another example is a town in Fallout 3 where several cultists worship a massive talking tree, this entire town is covered in leaves and huge branches. These unique cities gave these games character and made them more interesting to explore. Bethesda doesn't even need to go back to how the old Fallout games played, just bring back the atmosphere of the older games and the majority of fans will be happy. But honestly, that probably will never happen.