Its not the thinness that causes it to be slippy, its the sheer size, its ridiculous and has a poor chose of back material. I can't believe you're blaming the slippyness of a monstrously sized portable device on its thinness.Forget a thinner device. How about a bigger battery? The iPhone is too thin as it already is. I have to use a case with my 6+ just so I can get a good grip on it and not feel like I'm going to drop it.
If they are going to make a thinner iphone they should have Matte grip at the back of the phone so it doesn't slip out of your hand.