Member since 2011 and a paid member since soon after that...and this has been the number one feature on my list for years.
Specifically I would love the ability to encrypt an entire notebook.
To ease concerns over the data being stored in a cloud anyway, it would need to be encrypted locally with the private key generated and stored locally so that it is only in the control of the user. These encrypted notebooks do not need to be ocr'able or searchable.
I've tried various other commercial solutions and it just sucks having to use two options, especially for EN since I'm tempted to switch over all my non-sensitive data too.