Get rid of the shoulder buttons, call it "last gen".

Re-release the controller 18 months later with buttons back, and make BILLIONS.

Like a "New Coke", so to speak.

Thinking about it... was the Sixaxis/DS3 thing an accident... or an intention to make more money out of people?

Make it bigger and more ergonomic.
It's quite uncomfortable in its current form.

I just would like them to make it a little bigger or just have two version. One with the same dimensions as the DS3 and the other slightly larger. Just by 5-10% not much. Maybe triggers also since most people want them.

i wouldn't have a touch panel at the back like the vita, maybe even using the vita as a controller like the Wii U

No need for any major changes, it's been a synonymous gaming controller for over 16yrs now. Personally, if they'd wanted to drastically change the controller it would've happened a long time ago.

Hmm, assuming they actually make a DS4.

Much larger - GC controller size. With handles as long as a hand is wide.
Left control stick should be placed in a natural place for the thumb, again, like the GC controller.
Proper shoulder buttons would be nice, ones that push in instead of rotating.

Yeah... Why doesn't Nintendo release a Classic Controller? It boggles my mind!

*Something I forgot:

Add some "rubber" grips so the controller is more comfortable to hold, and prevents possible "sweaty" hands.

Concave analogue sticks.

The controller is almost perfect in my opinion. I love the placement of the buttons/sticks. The D-pad is one of the best around. And the shape fits perfectly in your hands. The only thing I could think of is making the sticks and L2/R2 buttons concave and maybe slightly longer handles.