LibreOffice is great for office documents and mainly what it was designed for. It is very close to being a good journal program also. The features that are needed will be…
-Dates
Have a calendar with each date being able to type what happened that day. This will include a title for the date and the description.
-Location
Allow users to add locations to mark where they were that day. You could add Google Maps or OpenStreetMap to do this. Also allow multiple locations to be marked each day.
-Tags
Allow tags to be added for each date.
-Encyrption
There is already an encrypted document features in LibreOffice but to ensure this feature is also for journals.
To make the files scaleable it maybe best to save each date as a file and save them in folder or compressed folder with password if the journal is encrypted. For example May 1, 2018 will be named 2018-05-01 and saved in the James Doe folder or James Doe.7z folder. The folder or compressed folder is the name of the journal.
The best two ways to achieve this is to either make a new program like LibreOffice Journal (Instead of writer or calc) Or add these features to Writer and only allow these features to be saved in a new filetype or as a .odt filetype.
I have tried many journal apps and they all lack in some feature or are discontinued or very unstable.
This is just a feature request nothing more.