I love this idea. Threema is a nice application, but has been stagnant for years. I use to at both work (Threema Work) and home for our family - but I've given up hoping for multi-device support.

That being said, a pure Proton end-to-end messaging app would be great too. I'd like something that it well integrated into the rest of the Proton ecosytem.

Good idea but I wonder just how secure this would be. Rumour has it that even Signal has been compromised.
Could then be a quick messaging service within Protonmail itself ?

Threema's 'Partner Program' actually is what more usually is called an Affiliate Program. IOW, it's for businesses selling Threema products on, as it were, on their own site/s, so of zero interest or value here.

This part of their site would seem to indicate that Threema have a decent foothold in the corporate sector. So it's actually very doubtful at the least that some sort of integration of Threema in Proton would be feasible - for either side.

However, some kind of E2E encrypted IM plus voice chat app - with the possibility of eventually adding video chat - is definitely highly desirable and I'm surprised at the low number of votes so far. Another point is, unlike Threema and some other messaging apps, any Proton app should function purely over the web and not require a mobile connection.

Would it make sense for Proton to be a Threema partner? https://threema.ch/en/work/partner-program

A messaging app from Proton would be awesome. Signal would also be a good option. If you remove the crypto currency nonsense from it.

That would make sense, rather than Proton having to re-invent the wheel from scratch. Also, Threema user base is quite small still, so it probably would work out to everybody's advantage as integration into Proton would be almost guaranteed to greatly increase user base very quickly.

Thought it would be a great idea to collaborate with the Swiss based company who develop the secure messenger app Threema, to somehow integrate this into ProtonMail.