A related idea,
Kagi supports long retention for chosen conversations, however the more conversations you have the harder it becomes find the content you need.
I would like the following 2 capabilities:
1) Tagging conversations (using custom-made tags), and filtering conversations by tag.
2) Better search, the current search provided searches only conversation's title, I would like you to index the entire content and search (you can open it to ULTIMATE members only if it costs too much).