What do you think about an upgraded display ( pixel density and resolution ) of the new Mac? For me this is the biggest argument of being a success or not...
Nah. Retina will be standard and unavoidable in a few years time. That doesn't make it necessary on every product right now. I don't see how they could put a 4K display on the 27" and not charge $5K for it. If they achieved something amazing with screen manufacturing and costs, I'm all for it, but if the computer cannot be purchased by mere mortals then it also will not do well.
I DO expect it to have lower glare and perhaps better color accuracy. That speaks to screen quality without increased resolution.