I think the toolset and general workflow of GS3d needs most work. More macro's, small sed templates for complicated stuff (small stuff so it doesn't get outdated fast), making that ctrl+space sed windows bigger. More and updated shaders, including better shadow shaders and terrain shaders. Better debugging tools. Easy to set-up windows dialogues (would be nice as extra flags/functions for panels). More modern version of and less buggy MED. New WED.

Models being able to split the world in manageable chunks just like blocks can would be nice too so blocks are not needed for that anymore. And other stuff to increase performance.

Linux and Mac support is nice for people who want to sell their stuff (+ ~10-15% potential sales ?). I don't know how much time it costs to implement though. Android support would also nice but more for fun stuff than serious stuff I must admit.

Small features are always welcome too and they can be faster implemented by the devs so that's a plus.