2nd monitor used for non-work items
Many people have a second monitor. In fact, the MDI-to-SDI shift was initially explained as fixing a problem with such.
I propose Excel enable a work display in which ALL, and I mean ALL, non-grid items, except the obviously needed scrollbars, are moved to a second window with the idea being for most people to place the grid-only window on one monitor and the tools, navigation, etc. items window on the other monitor.
So you have a maxed out grid, or several of them if several files are open on one monitor and the Ribbon, tab navigation, helpful calculations and reminders about CapsLock and so on, menu, that stupid Search box, and so on in the other window, presumably usually kept on the other monitor.
Not only would this offer max workspace without compromise, but many of the other items, such as the tab navigation tools, which are limited by needing to take the minimum of screen space, could be improved. If they were presented in small windows of their own in the program window, they could be resized to suit needs. (People would fall over themselves running to market with add-ins and macros to ease set-up and changes between several useful set-ups for a person.)
When I say several grids if several files are open, naturally one could have several fullscreen windows piled on top of each other as well if there is no relationship giving one a point to sharing space so two or more show at one time.
Also possible would then be using both windows on one monitor, sharing or not (if not, taking on the overload of clicking back and forth, so...), if one were so minded or only had one monitor (laptop, tablet, phone?). Excel could come with a default presentation for that frequent situation that was very close to current presentation, but with the better tools available to scrooll through or "blow-up" when moused to, receding once something's chosen.
I think it would solve a lot of problems and would actually be hugely freeing to the Excel folk vis-a-vis future growth and solution of longstanding problems.