I think what made the GT look bad was the fact that they made it a little taller than the F10, and also that they made the rear somewhat high.
BMW should just lower the GT's roofline to the F10's height, which will automatically allow for a lower rear, and accentuate the coupè silhouette a little more, the car would look much better stock, esp. w/ M Sport.
The GT's front is not that different from the F10's, besides larger kidney grilles and more different headlights (of which I actually prefer to the F10, since they are angled upwards and look more aggressive, rather than the little slant downwards on the F10's headlights) so that should be left unchanged.
The GT is based more of the F01, so it shares more proportions with the 7er than the F10. The GT is over 2" wider than an F10, bigger rims and tires (same specs as F01), has plenty more interior space, panoramic roof (honestly this was one of my favorite little features), and the dual opening trunk is pretty useful in different situations.
The GT should have really been a 7er, since they have more in common. I think the F10 is a great car, but when you sit in the back of a GT, the F10 cannot compare whatsoever, and I know many do not care that much about rear legroom, but it is the perfect family car, and you can be comfortable all throughout long trips.
By lowering my GT, I think it has significantly improved the look. You can see in the pics below that a lowered GT doesn't look any taller than a lowered F10 (@Stevo_N63's ride).
However, if the BMW designers follow the suggestions I wrote above, the car will look a lot better from the factory itself, let alone lowering it then.
Salt & Pepper: F07 & F10 by
dinan_gt, on Flickr
Salt & Pepper: F07 & F10 by
dinan_gt, on Flickr