Are you content with a whisper that everyone who can see whispers can see? Or are you asking for an e.g. whisper to self feature? That I find kind of interesting. I often set reminders for myself but then don’t remember anymore why I set the reminder.
There are other contexts where whisper to self is potentially interesting, e.g. when you assign/unassign a topic from yourself or close a topic.
If you’re happy with everyone seeing your whisper, I’d personally suggest you take the time to write that whisper in addition to setting a reminder for yourself.
That said, an addition to the bookmark system to also display notes with bookmarked topics as well as in the bookmarks list might also be useful. Just like we have link backs to topics linking to a topic or post.