This is such an unimaginative take. There's are plenty of things they can do.
- For a start, they can go waaaay smaller than 3nm. It's gonna take time, but there's a long roadmap to sub 1nm processes floating around somewhere (if it hasn't already been shared).
- There's lots of room to move with the camera too. Optical zoom, experimenting with more physical features like ND filters, electrochromic glass or similar for underscreen cameras.
- Underscreen touchID.
- Lots of room for improvement with battery tech (solid state batteries, faster charging, reverse charging, etc).
- Custom chips for specific purposes. They have the neural engine, ultrawideband, and all sorts of coprocessors, they can basically keep working on more and more of those as they have the need.
- Display tech can always get brighter, more power efficient, dimmer, higher refresh rates, better colour reproduction.
- Cosmetic changes. You say there's nothing left there because you're stuck in today's view of what a phone is. As things change, the role of the phone might change, meaning a change in form factor that we can't predict right now.
- Repairability. People like to make fun of this but an iPhone with effort put into being easily repairable by the end-user (without needing to rent 1000kg of tools from Apple) would be huge.