The standard desktop form-factor has not changed much in 3 decades. If you look at the standard PC tower its basically a 1980's IBM PC turned on it side.
Any of the more "innovative" designs over the last few decades never really took-off or lasted. I believe Apple's new Mac Pro could start a "fashion" trend in the desktop PC design because its so different from most other case styles available from PC manufactures.
If you have not seen Apple's new Mac Pro case check out the video below as an example.