Specifically I wanted to know which apps would be able to communicate with WhatsApp

WhatsApp will allow any service to communicate with their network. But wether or not any do is entirely up to those other apps. I think there’s very little chance Signal will ever interoperate with anything for example. iMessage surely won’t either.

Technically it shouldn’t be difficult, because almost every chat app these days uses the same protocol (Signal which is an unofficial industry standard and soon to be an official one). The question is how well it fits with their business model. And most companies don’t share their business model.

The other issue is the recipient needs to opt in. You won’t be able to send messages to just anyone… and if spam is an issue then everyone might turn it off.

The bigger question for me is wether or not you will be able to use a third party app to access WhatsApp. As in full access, view all messages, view contacts, create messages, receive push notifications, etc etc. It looks like the DMA might allow a return to software like Adium which is an open source messaging app that used to be able to log into almost any messaging service. These days none of the most popular services are available in the app, so almost nobody uses it.