I think it would be very useful if I could create my own opening database. It would be just like the two existing opening books (Masters and Lichess), but created by me. It would contain games that I have played, as well as other games I choose to add to it. I'm picturing that there would be a button next to Masters and Lichess in the Opening Explorer, and it would act similarly to those databases. It could be private, or maybe I could decide to share it with others.
I personally would use it to contain my repertoire and/or interesting games I've come across, so that when I analyze a game, I can see what move I intended to play in that situation.
I'd also like a similar feature. As a starting point I'd love to import my own games as the database used for display. That and red/green (good/bad) arrows for my most common moves in typical openings would help me improve tremendously.
The analysis tool provides some of that but I've a hard time understanding the opening names and where I typically go wrong.
A personal opening book is expensive. Both in terms of storage and in terms of computational power.
For AtomicChessBot when I compiled the book of all time >2200 lichess games, it took me half a day. The point is that parsing PGNs and translating game moves to fens is not that trivial. Also the book is 16 MB in size. Imagine tens of thousands of lichess players having tens of MB size books. We soon are talking about hundreds of GBs which even by lichess standards is a lot of storage capacity.
I think you should not expect too much on that front, and this is not due to lack of willingness to write code on the part of lichess developers.
I didn't know about the FEN vs PGN before but it makes sense that the database would have to be different to allow game stats and quick lookups to occur. That being said 16 MB isn't all THAT large of a storage requirement even for 10k people. Even the option to upload my own FEN files that would only stay on the server for a day or so would be super useful. That would at least remove/reduce the processing of PGN to FEN files and storage requirements of keeping/maintaining them.
Get SCID and create as many databases as you want
It's not the database but the user interface that lichess has for the opening explorer that uses a database that would be a nice feature to have. The opening explorer interface is really slick and easy to use. I'd learn so much about common positional mistakes I make and I think others would too.