I would like to propose something similar to Teams, where an organization can introduce Teams and in these teams, groups can be made for instant messaging, one-on-one messaging can be facilitated and files/folders from Proton Drive can be accessed without changing the application - integrated with the existing Meet ecosystem.

Having this would be a tremendous way to boost how employees could interact with each other and make very efficient cross-application use of the Proton platform at the same time, integrated with their workflows.

It would also be a really good way to advertise Proton as a viable, strong competitor to existing Big Tech solutions in this space, where I personally see a lot of potential for Proton to grow as a business.

My suggestion is to, if possible, base this chat platform on the XMPP protocol, since existing end-to-end encryption protocol-level integrations are already present in that ecosystem.

An alternative would be to use the Matrix protocol (as long as precautions are taken to minimize metadata leaks as much as possible), since the Element messaging/collaboration app has been built on top of this - it is FOSS software and could be a strong candidate for facilitating the backend of such a project.
This may be worth looking into.

I'd be happy to see others give feedback on my idea!