The obvious answer would be some sort of consumer AI assistant, like Siri but better.
And what exactly is that device going to do that the iPhone (and smartphones in general) can't already do with at a minimum a few small tweaks to the existing flagships?
In the right context vertical integration can make sense, but hardware is a big stretch for OpenAI right now. They haven't even really pinned down the consumer software angle yet.
10 mins I spent watching that weird video in third person dialogue no less, they better deliver something great.