I am not sure if this was sarcasm, but I believe big data was already yesterday?

Not sarcasm. This is more a reaction to big data. Here's an analogy: Imagine cloud providers like iCloud, Google Drive, OneDrive etc. As a reaction to those, Owncloud and Nextcloud emerged for personal (well, also business) use.

My idea with this is inspired by that. It's just for personal use and to address my own needs.

eh, a hint. i was digging around some thing in these veins long time ago - more like collecting one's notions, not exact low-level actions - but apart of it being impossible back then, i dropped it for this simple reason: if you build such thing, it will know about you much more than you know. And that, in somebody else's hands.. identity theft would seem like walk in the park.

For sure, thank you for that hint. One of the most important things to consider is that something like this can't be misused on someone else, e.g. as a surveillance tool.

I should have clarified, I'm only building this for myself and my own use, there are no plans to take it further than that. Basically, I am trying to learn while building something that satisfies my own needs.