A more sane expression syntax might be nice too, but isn't necessary to make Excel more productive for abstract programming.
For a feature like this to be successful imo it needs zero sandbox concerns; a user should never be prompted to downgrade to a lower security configuration, it has to be built in and available by default in every sheet. I don't see VBA ever being safe enough to enable by default everywhere. VBA is just an escape-hatch to a 'real' programming environment; my claim is that excel sheets & formulas alone could be a 'real' programming environment in its own right, no escape hatches necessary. All just by adding one feature to the existing formula language. The reason why excel is so popular today is its simple and understandable expression language and a unified spatially-oriented layout for code and data. Just shoehorning VBA into a cell loses all of the understandability of the simple excel formula and brings with it all the baggage of the old way of needing to manage source code. I think we can do better than that for normal everyday excel documents and normal users.
function (x, y) { return A1*x + y; }