I'd pay a lot for a desktop version of whatsapp.
I live in Asia (Hong Kong) and 90% of my communications with clients, staff, partners go through whatsapp because its instant, free and everyone has it. But I'm really tired of having to pull my phone to reply to people when I'm on my computer.
It's a $19bn company, someone should work on this, ideally something multi-platform a la Slack would be fantastic.