Vancian magic.
Honestly, I think most of the problems of pre-4e (D&D) were due to LFQW. That is, we can fix it by changing how powerful spells are, how they scale, number of spells per level, etc.
I got very sad when 5e was launched and Vancian magic wasn’t there. I met 4e before Vancian magic, though.
Anyway, I get that many people don’t like it, but I believe the main problems are more related to LFQW, how powerful spell-casters are in named levels, lack of options in lower levels, etc. Maintaining the essence of Vancian magic and balancing (by changing the rest) is something that, I believe, it’s not only possible, but also has already been done; as in 13th Age, for an example.
I also consider Vancian magic to be very flavorful. Forget 3e’s preparation; I use a more old-school concept of memorization. I read about it in AD&D (1e-2e) books and eventually wrote the concept with my own words. It really makes me feel the “magic” in the world, even if 25% of all people are wizards! Hehehehe

Honestly, I wish I could just ask people about what they don’t like in Vancian Magic, and treat those problems within Vancian magic, without starting a flame war and having the thread banned, people calling me bad names, or something worse than that. One day, perhaps, when peace is stable and the wind roars through the skies.
There are some other mechanics I like, but I’ll leave this post just for Jack Vance.