Yup, would have wished for a 787/777 style overhead panel, a real EICAS including modern style electronic checklists (again, 777/787 style), an approach mode that can follow curved approach paths (RNP AR/GLS) or simply a new auto flight system.
And of course more room, but i gave that up a long time ago.
The above commonality move to the newer boeings would have helped marketing a true MFF solution, which airbus for example can. Yes, we have already an approval for 737/787 MFF but that is rather awkward with the huge differences between both types.