Make everything faster. Specifically, auto-retopology and 4K painting. I think the main thing that is holding Mudbox back is slowness compared to other programs. If Mudbox were blazingly fast at what it already does, I don't think it would necessarily need any new features.
That said, dynamic topology would be useful.
Smudge brush for painting.
True sculpting symmetry that doesn't gradually stop working over time.
Mirror joints.
Being able to bake more textures. Like, edge mask, instead of just ambient occlusion. For use as masks during painting. Like smart masks in Substance Painter.
Something like Blender Metaball would be cool.
More sculpt brushes.
Better hotkeys. Not just "1,2,3,4, etc for brushes."
Better interface. Easier to snap windows around.
