With mail you can create an URL like mailto:john.doe@example.com?subject=The%20message. If you click that link, an e-mail message editor will be opened with the provided emailaddress and subject.
With WhatsApp you can do the same: https://wa.me/1234567890?text=The%20message.
It it also possible to do this with Google Hangouts, so it opens a chat with a provided emailaddress? When I check de developer documentation I only see REST documentation for bots and such...
Take a look on this case: https://webapps.stackexchange.com/questions/58767/create-direct-link-to-google-hangouts-chat
The final URL is:
https://hangouts.google.com/chat/person/1XXXXXXXXXXXX
As you can see, you need an ID of the person, 1XXXXXXXXXXXX and it is not person@gmail.com format.
If you want you can file a Feature Request following the next link, requesting alternative format(email) for an ID:
https://support.google.com/a/answer/6284762?hl=en
I hope this solves your doubts.