I've always thought about just making a chat client app using email as the underlying standard. Strip out CSS and signatures (and funnel spam/marketing to a separate UI) and bam, you have possibly the most widely-interoperable chat client ever made.

The ostensible "problem" with this would probably be "email is too slow for real-time chat!" But really, how much difference does a few seconds make? And modern connections/tech make me think that idea might have to be reevaluated anyways.

It wouldn't be a business but I could see such an app becoming quite popular.