IMO

-Longer fatalities
-Throw on some slow motion camera shots and more camera angles(some say the slow motion thing is played out but it isn't in movies and and definitely not in vodeo games)
-Better looking blood
-I would have to say one of the best fatalities in MKD was when Sub Zero freezes your bottom half then comes up kicks your top half off and then slowly walks to you as you scream(nice add showing the fear and pain) and then he stomps on you to put you to rest.

Here's a very simple old fatalitiy but done right could look great! Take Kung Lao's old hat fatality from MK2 where he slices you in half. Have a camera circle around him and the oponent he takes his hat off and in slow motion he slices the guy in half with some really good shots like the camera followong the hat down up close and really good sound effects then after that show a high angle shot with him doing it fast and another close up of the face with blood seeping through the skin and a mid to long shot of him either falling in half showing the insides in good detail or the person taking a step then falling in half.
In my opinion a simple fatality but when done right a masterpiece.