start using C# as the macro language instead of visual basic
well, visual basic great language loves it but Microsoft seems to have abandoned the language. C# is clearly the new emphasis so, therefore, start the move to make C# the default language for the office products. plus there is an opportunity to increase the performance and features of the macro language. allowing for the creation of exportable modules and even binary macros that run faster or can be more easily exported to different machine
As long as there's an automated migration tool for existing VBA to be migrated to C#. Businesses otherwise would be very upset if the legacy support was dropped. I would tend to prefer VB code purely due to familiarity. But if a user can choose between, then great.. Add Python while you're at it, being a data centric app (you might then get data more scientists using Excel).
Plus bring over the objects and features of the modern VB form editor from Visual Studio over to VBA. Ie a 64 bit calendar, object anchoring to other form elements to ease resizing, modern property options for objects. I like to drag components into the form in a what you see is what you get style, then code the functionality etc, so a rich UI editor revamp would be amazing.
Also multi threading. Ie if a process doesn't strictly need to be sequential, allow the user to split a piece of code out to a background task (perhaps include a wait option for the following item in the stack for optional use). Like workbook event triggered processes, allow the user to assign them to a background task, to give the Do Events style of functionality (of not seemingly halting the app until it's finished the VBA process), so that new events and buttons can run, while the previously triggered background task is still running.