Looking at the new features poll, I don't see anything on there that I think should be tackled first.
I'd like to see general improvements to GL Golf before those things are added. The physics are much better now (but they could still be tweaked), and the general gameplay is good. Overall though, it's lacking some polish and detail. Things like:
The textures and environments could do with a bit of work.
Different species and height of trees would be good (e.g. oaks in the english countryside course, and cactii in Nevada, rather than pine trees for all <g>)
Different lies and effects from those lies (in the rough, bad lie in the rough, ball half buried in the sand, etc)
Stance positions for fade/hook, and special shots like chip/punch shots.
And something for an option - a mulligan, the ability to redo a shot if you screw it up badly. Perhaps making it so that if you do enable that, you can't send through your scores to the high score list.
It's a currently a good game and fun to play, but not yet a great game.
