Based on what I've seen, one of the main uses of spreadsheets is financial management — both for personal expenses and investment tracking. I use Google Sheets a lot for that, and I was really excited when I saw the launch of Proton Sheets. However, the lack of a feature similar to GOOGLEFINANCE — which works for securities from multiple countries, not just the US (I use it for Brazilian securities, for example) — is a bit of a letdown.

I believe this would be an important feature that would make it easier for many users to migrate.
As an additional suggestion,

I'd also recommend improving the documentation for your services. In Sheets, for example, the only help documentation I found about formulas is this one (https://proton.me/support/sheets-formulas#start), which lists only a handful of functions — even though the app supports a much larger set. This forces users to figure out on their own what's available and how it works. For comparison, if you search "Google Sheets function list" or "Excel functions list," you can easily find everything that's possible to do, along with examples of how to do it.