If I couldn't ditch those ugly jackets altogether, I would've had them worn only as dress uniforms; everyday fatigues would've been just the turtlenecks, like the pilot uniforms. And I would've rather seen them in deep blue instead of brick red. The belt over the jacket doesn't have much point either. (Edit: Apparently J.T.B. and I are on the same wavelength.)
Simpler insignias and rank pins too, while we're at it. I don't see the point of that oblong shape behind the arrowhead/circle.
Aww, hell, I would've kept the TMP uniforms, just with somewhat richer colors. Those uniforms had a lot of variants that made them seem far more practical and realistic than other Starfleet uniform designs, and they didn't look like something out of a historical costume drama. Maybe crossed with aspects of the Star Trek: Enterprise uniforms, like pockets and embroidered mission patches with writing on them.