I am not sure if I know what I am talking about or if it counts in this scenario but constraint solvers come to mind. I am mainly familiar with them in a CAD context so I am struggling to think of a use for them in a spreadsheet context. But I think being able to say given these endpoints find me some values that fit could be a very valuable tool.

But like I said I am not sure that I know what I am talking about and I may be confusing backwards calculation with algebraic engines. I would love for algebra solvers to be a first class object in more languages.