ISTR seeing illustrations from an Apple patent showing a computer display with a slot in the back into which an iPhone can be docked, turning the phone into a full-size desktop workstation.

And this was during the Jobs era.

Apple pays their employees to file patents for pretty much anything.

That doesn't mean they will ever manifest in real products.

How do you figure? The iPhone was unveiled as a 3-in-1 device during its first keynote.

The iPad is already this but with a bigger screen.

No, I'd argue it isn't. Even if you connect an external keyboard an display to the iPad, you are just getting the ipad experience on a larger screen. There is never any transformation of interface when used this way. Apple has never allowed mouse/trackpad on iOS(with some very very narrow exceptions) so it's not transformative - you are still getting the same experience essentialy.

For it to work on the iPhone, they would have to add support for some kind of mouse/trackpad, because the iphone itself is sitting in the dock - and I don't see apple doing that in a million years.

iOS still has a long, long way to go. It seems like they're preparing all the hardware, though.