Open-source the visualizations engine in Excel like you've done with PowerBI.
I'm going to paraphrase Chris Campbell from Blue Granite because he sets it out far more eloquently than I ever could:
The trend for the past several years has been for customers to want to leverage out-of-the-box capabilities of their BI tool to support reporting and analysis. But software companies simply can't keep up with new innovations in visualization.
Open sourcing visuals in Excel means they would become drag-n-drop objects just like all the other built in options. The ability to extend the capabilities of the product with an easily accessible, open source interface opens up tons of possibilities, and gives Microsoft an army of free talent from the community at their disposal.
When faced with a challenging need, advanced users (or their development teams) could build exactly the right visual for their needs, practitioners can build portfolios of their own work to differentiate them from their peers and a widely accessible, open source community that leverages a well known and powerful platform means that support and examples are only a search and a click away.
jeffrey Weir commented
OMG! Microsoft did this already. https://powerbi.microsoft.com/en-us/blog/excel-announces-new-data-visualization-capabilities-with-power-bi-custom-visuals/
Didn't office.js provide this functionality?
This would be amazing!