I would be very interesting for a "plus" type user of protonmail to creat a folder with a pre-defined maximum size on the drive that can be shared (download + upload) with free account users.

Sharepoint is a key feature when it comes to organize or work with people. It would be a good incentive for people to create a free account. I struggle to move people to protonmail, they feel concerned about privacy but not enough to take action. If stuff they need is only accessible through a proton account they will make the first step as it is free (by the way it is what google and the others do.. need a gmail to operate my phone, need a gmail to register to an event here or there because the file is on google drive....). A fraction of those will be seduced by the privacy and transparency proposed by protonmail in general and they will be one online payment away to ditch their old gmail address for daily use.