Two-way cell connections
Looking through 150 pages of suggestions here, it's obvious that many people want and could usefully use "connected" cells: two cells (or more, and perhaps two (or more) columns, not cells) that contain no formulas to overwrite when entering data in them, yet each must remain slaved to the other.
For instance, A1 and B1. Either may take an entry and the other must change appropriately. Perhaps B1 should always be three times A1, yet these two cells are to take data entry OR have other dependencies created in them with formulas. In the former case, enter 5 in A1 and B1 should immediately change to be 15, or enter 9 in B1 and A1 instantly change to be 3. In the latter case, B1 might have a formula referencing Z3 and C109 and if either of those two cells change value, B1 of course changes value, but then so does A1 without A1 being in any other way aware of the cells B1 is referencing. It might even have its own formula that references completely different cells, who knows what eventual use could occur here. And those cells might have their own two-way dependencies.
But really looking for the relatively simple functionality here, at least to start with.
Perhaps it might be accomplished in the Name Manager or something similar made for it. One might specify the two cells, then the relationship, perhaps like this:
=A1,KD8345,"A1=3 * KD8345"
or slightly more involved:
=A1,KD8345,"A1=3 * N75 / KD8345"
In any case, there are many levels of users of Excel. How this might be "properly" done by "using good programming standards" isn't the point. Workarounds aren't the point. Just having the simple, quick to use functionality is what many people want. And not only people who neither wish to know nor do know how to "properly" build such functionality. Lots of times one wants quick and dirty (if need be to achieve "quick") usability to perform a task for people who have very different professional and knowledge orientations.