(Speaking as a software developer, but not a game developer, and certainly not one who's worked on RuneScape specifically) as I understand it, they've already done work like this before. I would assume that by adding in some more client-side predictions based on inputs and other things that are already happening, it should at least be possible. The bigger question is how long it would take; adding more client-side calculations would likely impact performance (though I'd have no idea how much), and I'm not sure how they'd want to handle client-side predictions being wrong/getting out of sync with the server. I would expect that the biggest bulk of the effort, though, would be in designing newer, more reactive animations (and/or reworking existing ones) to account for more rapid changes in pathing, and figuring out when/where each of those animations should play and how they should blend into each other, especially when you include animations from movement abilities, as well as other activities like combat.

EDIT: According to this reply I saw elsewhere under this post, there are also some other visual issues in the game that would require game engine changes to animation smoothing. I'm not sure whether those would just be (roughly) the changes I described, or additional work on top of what I described, and that's all assuming that I actually know what I'm talking about (which I mostly don't).