Looks good, but feels wonky when you actually use it.

When I type =SUM( in cell A1 and hit arrow right (before hitting enter or tab), I expect the address of the formula to change to =SUM(A2

F2 should work for editing formulas instead of actually entering "F2" into the cell's content

Many other little "expected behaviors" need to be implemented for this to be a frustration-free experience. You should get a team of experienced Excel users involved so you can see how they work / or they can tell you how they work