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.

To keep my mail and cloud data both in and via Swiss I use Protonmail and Tresorit cloud storage.

While it would be great (damn google's prying eyes)... I say no, focus on email.

I agree with that they should focus on email.

Would like this as well

I'd like to see ProtonMail stay focused on Email. No need to be a jack of all trades. SpiderOak is a good place for encrypted zero-knowledge online storage if you want that.

It is an excellent idea!

Provide secure alternative cloud file storage. I do not trust current providers and they are also easy pickings for law enforcement prying.

Please add cloud storage for files like Mega or Sync

It would be great if I can store all my Gdrive or Dropbox data on a secure service, like yours.

I would love a feature like storage even without a client site software the same kind of security like the mail service i think it would defiantly attract more people to this amazing company !
Even if it where just for personal storage not even for sharing purpose

Cloud storage, reliable address book, calendar and sync from/to mobile devices (Android, iOS, BB10, WM)

Shared drive for securely storing documents, files, etc. This would also encourage more people to upgrade to paid account

I would much rather have ProtonMail store the data than have it stored on a 3rd party cloud service, even if that data is encrypted. With the US government's PRISM program, all files you upload to services like Google Drive or Dropbox are accessible by the FBI and NSA in real time. Also, the cost of storage space for ProtonMail likely isn't as high as you might think; most likely it's everything else like employees, hardware, and facilities that are ProtonMail's main costs. It's unlikely that storage is so extremely scarce that ProtonMail would have to integrate with 3rd party cloud providers just for storing attachments.

Ultimately, nobody should be required to sign up for a Google Drive or Dropbox account in order to send/receive email attachments in ProtonMail. Also, if you need more storage in ProtonMail, paid accounts are now available; they're not that expensive and the money supports a worthy cause.

Cloud storage would be great, but...

Uhh, I have no idea what you're talking about with a "secured window" and "extension of e-mail view"...?

Also, 4 or 6 digits is not at all secure, anyone could iterate through all possible numbers 0000-9999 in less than an hour and break in.

I would love to have cloud storage like dropbox from proton mail with added feature which no one currently has is the added extension of E-Mail view mail with 4 or 6 digit privacy protection in a secured window.

You could use SpiderOak. They're in no way associated with Proton Technologies (and hosted in the US), but also apply a zero-knowledge encryption system (only you can read your data in plain text).

DropBox like storage

Maybe mail attachements could be stored encrypted by protonmail on a users cloud storage space to conserve protonmail serverstorage, costs and possibly increase reliability for the storage of the text in the message.

The chosen cloud storage could be the users own server like a Synology workstation cloud or some other cloud storage service (dropbox, google or whatever) set with appropriate access parameters to the encrypted data in some way in your Protonmail account.

Obviously data loss of attachments could happen if this is allowed so user should be warned.

Time limitation for mail access should work. Protonmail should try to delete stored attachment mail info if set expiration date is due.

If cloud is not reachable any more (due to for instance disconnection) all protonmail "keys" to be able to decrypt this information (after set expiration date) should at least be deleted.