You can use most cloud storage services and even encrypt local folders, all automatically, by using Boxcryptor. Have been using it for years on Dropbox, Box, Google drive an others and have no complaints. There is a previous version still around for which I bought unlimited service at the time. It is now called Boxcryptor Classic. Look for it on their website.
Although only guaranteed to work with Windows up to V8.x, I have experienced no problems in Windows 10. Still works like a charm.

I would use Proton Drive, Proton Photos, Proton Calendar, Proton Messenger, and Proton Bookmarks.

This seems to be a duplicate thread.
https://protonmail.uservoice.com/forums/284483-feedback/suggestions/14880600-need-to-have-proton-drive-where-i-can-store-all-my

Their TLS certificate already has protondrive.com added, so it seems that it is coming:

DNS Name: protondrive.com
DNS Name: www.protondrive.com
DNS Name: secure.protondrive.com

protonvpn.ch is also added, which means they are probably working on a VPN service as well?

DNS Name: protonvpn.ch
DNS Name: www.protonvpn.ch
DNS Name: secure.protonvpn.ch

Please don't do this :-) Secure storage is complex topic. You don't want to get sidetracked and spread yourself thin. Focus on email. Secure storage is something Tresorit and SpiderOak do very well.

or mega.nz

Use mega.nz
q:

That is a very good way to promote your premium plans.
I would pay for a combined email/storage space without a doubt. I still hesitate to do so just for email.

I would like this since I prefer having these kind of services in the same place. Especially since the file storage could integrate into the email part when sending files.

Also, I don't need 5GB of email storage, so that is just a waste for me, which keeps me from upgrading. But if I could use that for file storage, in a sane way, then I would use it!

Yes please! I haven't been able to find many good Swiss cloud services for consumers. Most of them are aimed at enterprise, and if they do have personal plans they only have low storage options.

This would be a cool feature.

Khm...Khm... iCloud Drive.... Kim...khm...

For secure document storage see Tresorit as an option.......

search for tresorit.com, it is exactly what you are looking for ;)

I've given this one vote, because even though I think it makes complete sense for ProtonMail to focus on secure email, I'm already using ProtonMail as a defacto secure storage technology for certain items. It's the only platform I really trust.

Agreed with the comment consensus here - email is what I'm looking for, not document storage.

tresorit has good features(zero knowledge mobile upload, browser) that spideroak doesnt(mobile and browser is not zero knowledge). spideroak has a good priced unlimited plan. If proton mail does get to this, it would be great to have a reasonable priced unlimited plan with more tresorit features. if it falls short in price(too high) or not enough features then users will have better options elswhere. would switch email and unlimited data backup to proton mail if they actually move forward with it

While I would LOVE to see ProtonMail get to this, I think the priority at the moment should be email communication. Tresorit does a great job for encrypted cloud storage and is also Swiss based - I suggest looking into them at the moment.

This seems far outside the scope of protonmail's service offering, and would be better served by another provider. Its not hard to host your own Owncloud instance or similar with client side encryption.

Focus on email. You can just encrypt your stuff and then put in on Google Drive for the same result.