[gumby]

One advantage of using lisp is it would make it easier to develop complex routines. You could even import external code, so you could write/debug some hairy algos externally and then use them in the spreadsheet. Imagine having the power of macsyma and doing pdes in your spreadsheet. Or being able to do advanced vector operations on columns.

Though I am not as fond of them a stack language like forth or postscript could also be the computing paradigm of your spreadsheet.

Or if you are feeling more perverse, there’s always TECO