From a practicality standpoint, yeah, even slightly bigger next gen Joy-Con wouldn't reasonably house a trackpad, even a small one. I think the most practical option is a combination of your suggestions, haptic capacitive L and R for scrolling, capacitive ABXY for a pseudo trackpad. The patent includes measuring distance of the digit from the buttons, so it should be able to work like a normal trackpad using only four refefence points, pretty neat concept.
Personally what I want to see is capacitive pressure sensitive L and R, giving us scrolling shoulder buttons and analogue input, at least. But for a pointing device, I think they should bring back the 3DS C Stick.
And no, I'm not joking. A full size trackpad or trackball isn't realistically going to fit, and while building it into ABXY is impressive, it means a lot of inputs whether you mean to or not. The C Stick was a pointing stick, like TrackPoint, a fully featured mouse pointer. It's a tiny part with the input of a mouse, which is what we want. It can also come in different shapes and presentations, they could make it dome shaped and textured, like Lenovo's modern trackpoint, they could make it upright and cylindrical like the 3DS, and either way it takes up less space than a button.
In terms of layout, considering a pair of Joy-Con upright, I'd place it down and to the right of the ABXY diamond on Joy-Con (R), and while we're at it, add 1 and 2 buttons to the top right for a full six button layout. On the Joy-Con (L), it would be up and to the right, with its own 1 and 2 (1L and 2L).
This gives up a lot more input without a lot more space, and Nintendo has experience with this pointing device. Plus you have two, imagine what they can do with that! Shooters could use them as an alternative camera input for finer and faster movement, for example (Compare Minecraft New Nintendo 3DS and Nintendo Switch to see what I mean here, the C Stick is better for control in a first person shooter.)
What's important though is that this would also improve sideways Joy-Con play hugely. In sideways Joy-Con Play, you'd now have a nearly full suite of inputs. A left analogue stick for movement, the pointing stick for camera controls (whereas a built in trackpad on ABXY would he more awkward), and 1 and 2 help make up the difference from the lack of triggers.
So yeah, that's why I'd like to see pointing sticks (C Stick) make a return, to me it seems like the perfect balance. Small, familiar, mouse-like input that can be used in every play mode and doesn't interfere with anything else.
While I'm at it, I would suggest they expand the naming scheme for the buttons. They always call the analogue sticks Control Sticks, so call them CL and CR, call the pointing sticks PL and PR, call the 1+2 buttons 1L, 2L, 1R, 2R.
What this would mean is that every button would have a unique name even in a full controller setup.
At this point I think I'm dreaming up my ideal Nintendo controller, but the Wii U GamePad already exists.
