Auto-launching apps may arrive with Android P if a new set of strings found in the SystemUI APK for the first Android P developer preview is anything to go by. The strings make references to always opening something when the phone is connected to a particular device. The strings don't specifically mention apps, nor do they mention any specifics in regards to what device would need to be connected but the suggestion seems to be that when the phone becomes connected to an external device of some sort, it will launch a particular application that the user may have installed on his or her phone.
It isn't clear just yet if this type of feature would be configurable by the user, or how configurable it would be if it is. That said it's not hard to imagine the possibilities. For instance, this auto-launching apps feature could enable launching something like Google Play Music when it connects to a Bluetooth audio device, like the car stereo or a set of Bluetooth headphones. Likewise, the same feature could enable auto-launching an app like Netflix or Hulu when the phone connects to a certain Wi-Fi network, say the one at home.
At the moment the auto-launching feature is only visible as part of existing strings of code and is not something that you can actually play with, which means that it isn't ready for use, but that also means it is likely to show up in one of the later versions of Android P, whether it be a future developer preview or the final release of the software that Google pushes out to the public later this year. It's also worth noting that if it does show up in a future developer preview Google may choose not to add it in with the final build, or it could always modify it by that time. If this is something you've been wanting to see added into Android as a native feature, this will be something to look out for.