For gmail I'd like real imap and custom domain support. FastMail is the obvious choice, but I can't in good conscious trust an Australian company with my secrets.[0]
Hangouts: I'd like cross platform with a web client if possible. I would like optional chat history sync. And I'd like on by default encryption. Signal is great, but its treatment of chat history is too paranoid to be useful for everyday conversations. Whatsapp is also a strong contender, but I'd also prefer to stay off facebook services.
edit: for a hangouts replacement, I'm not looking for video/audio chats I'm looking for a direct messaging focused service that can serve as an SMS replacement along the lines of iMesseges, whatsapp or signal. And yes I know about XMPP I was a heavy user a decade ago, but I'd prefer not to replace a dying service with a dying protocol.
0: https://arstechnica.com/tech-policy/2018/12/australia-passes...