In tablet mode, iPad OS. Touch being the primary operation. Basically just as we see it now.
In a pseudo desktop mode, macOS, where you get the power of a laptop in a smaller form factor. You can optionally try to use this in touch mode in a pinch but it's not necessarily designed for it.
The win would be seamless switching. Including apps... if I have photoshop open on iPad, dock, convert to Photoshop for Mac. I.e. you "dock" your iPad and it converts to a more Mac-like experience. Undock, you get the iPad experience.
To me, this would be ideal. I don't generally _need_ a laptop for personal use, so this would be a serious boon for me as I use my iPad all the time in the evening for simple consumption, but I also have a MacBook over here that gets used a few times a week, which is a costly device for how little it gets used.
That phone is still kicking around my drawers somewhere.
The trouble though is, once you get an acceptable laptop keyboard and a corresponding screen, the resulting device seems excessively heavy and bulky when flipped over into the tablet form factor. You probably won't want to hold it up for more than 10 minutes.
I like the idea of one all-in-one device, but it's hard to see a way around these things. The cheesy tiny portable "keyboards" they make for tablets are pretty lame for extended typing, but better keyboards are too heavy for tablets. Meanwhile, I expect a desktop device to have the compute horsepower and RAM that are tough to get in a proper tablet form factor.