Copilot to be another.
Midjourney to be another - or at least diffusion based image editing tools which can be brought into photo and video editing workflows. The killer app here is probably integration of diffusion models into apps like Photoshop (and eventually video).
Some real virtual assistant applications seem right around the corner (i.e. a real life J.A.R.V.I.S seems like an inevitability within the year rather than a pipe dream, and to me would be a killer app)
And then lots of other killer apps are pretty obvious to imagine with development (e.g. customer service applications like IT helpdesks, Computer game dialogue where you can really influence interactions...)
But my not so informed opinion is text as an interface is only a small feature of bigger useful products, not the main focus. Instead of learning sql, you can ask a regular question. It feels like inventing the mouse to use with computers.
Video game dialogue remains to be seen, but I already find ChatGPT based text adventures super fun! So I suspect there will be demand for both handcrafted static stories and AI dynamically-generated stories (ie they can be different things, one doesn’t have to replace the other, just like email didn’t immediately replace the post service).
I don’t know if you enjoy copilot, but for me it’s definitely supercharges my productivity.
As always, the tech isn't the problem - the way business applies it is. Customer service automation isn't done to help you better - it's done to make it cheaper to make you go away without making too big of a fuss. Companies building and employing customer service systems will find ways to make even GPT-4 incapable of providing anything the customer would find remotely useful.