Traditionally, the melee advantage was being able to do damage while moving, but now a lot of ranged are also good at movement. We could certainly go back in and prune a lot of cast-on-the-move and instant spells from casters, but on the other hand we know players think those abilities are fun and you can even argue that having to stand and “turret” as a combat mechanic feels a little dated. There is also a continuum here: casting Lightning Bolt while moving isn’t a big balance problem, but something like passive Kil’jaeden’s Cunning might be.
Rather than making casters terrible at moving, we’d rather develop a niche that melee are really good at. For example, we could emphasize that melee are really good at cleaving multiple targets, or they could be more survivable, or both. We are going to explore these ideas more.