High Level Design Goals

Self-Hosted - When you join the platform, you essentially purchase a very low cost plot of land, so to speak, on the internet. This is a place where you are safe to store any kind of personal data you want, from your most sensitive data to things that you are comfortable sharing publicly. Data is encrypted both at rest and in transit.
Easy to Setup, Manage, and Use - However, all work to manage the self-hosting is done on the user's behalf in a reliable way. There is no technical skill needed. If you can sign up for an email account you should be able to sign up for this service.
Very Low Cost - The specific infrastructure that is provisioned on the user's behalf should be so low-cost that annual cost of running the platform per-user should not exceed ten USD a year.
Federated Content Sharing - Other users on the platform can be found and content can be shared with them. While you can never revoke access to content that was at some point sent to another user in the sense that they may have created a copy of the content, the platform will, by default, not store copies of other people's content indefinitely. Subsequent requests for content where a user has revoked sharing access will be denied.
Signed Content Sharing (an idea) - Clients on this platform will not accept content from a sender that is not signed. Accountability for actions taken in the digital space will be enforced as follows. Identities will gain trust within the system as they share content that is opened and is not reported as suspicious or malicious. If a user is being abused in the network with malicious content, they can block that identity. If the real world person tries to create a new identity to continue abusing the user, they will have a next to nothing trust rating. Any new contact attempts by the malicious user will be obvious and easily avoided.
Extensive Support for Content Types and Data Sources - The idea would be that this plot of land, so to speak, in the cloud would be the primary storage location of all content that is deemed valuable by the individual. It is a personal collection of your digital presence. Content that you could house in this space:
- Geo-location Data
- Money spending
- Search history
- Internet Browsing history
- Calendar
- Email
- Photos and Videos
- Journal
- Book reading
- Exercise
- Audio Recordings
- Music Listening
Basically, any contact with a digital device should be able to be captured and stored in your personal collection. This data can then be utilized in whatever way you like in order for you to remember or gain insights into your own behaviors and activities that will help you conduct your own life. You can also share as much or as little of this information with others as you like.
Baiscally, think of it this way: your phone will eventually grind to a halt and be replaced by a new phone. Your laptop will die. If you have a desktop, it will die. All of your devices are transient storage. Your plot of land in the cloud is forever.

Specific Approach

Cloud infrastructure is becoming increasingly cheap to provision. Also, there are multiple cloud providers, and they provide similar functionality. You could use Microsoft, Amazon, Google, IBM, Oracle... and probably others. Furthermore, cloud infrastructure often offers pay-as-you-go, fully-managed resources. Finally, it is possible to manage the provisioning and maintenance of cloud resources via scripts. What culminates is a picture of an open source project where both infrastructure-as-code and the software that runs on that infrastructure are packaged together and even the initial setup is done on the user's behalf. The end result is that the user owns their own cloud resources where all of their data is housed, and they don't even necessarily know it. Furthermore, the development and maintenance of the platform is done by a collective and is under public scrutiny. This is only beneficial to the security and reliability of the platform.