I highly doubt they will do a dual screen model. Perhaps for devs to test and see data on second screen but I think nintendo will keep the same form factor but I hope they add analog triggers because racing games on switch are awful.
All Nintendo needs to do to make tons more money is make a more powerful switch and that would be easy. The current chip in the switch is from 2015. Newer variants are available. They could also add more ram.
Honestly if they were smart they would switch to a ryzen laptop SOC. They should be able to maintain compatibility and they would get insane performance
I can imagine two plausible solutions with an external screen;
1: completely wireless, streaming a secondary image to the docking station via Miracast (or an equivalent protocol)
2: tablet-like docking station that you can put on a table and still see the screen, with multiple options for the second-screen display, such as wired all the way or streaming to a Miracast / Chromecast style receiver.