I would be a little surprised if they switched off of ARM. Even the Steam Deck with its custom AMD SoC is much closer to laptop specs and wattages than the switch e.g. 20w+ SoC, more than 2x battery capacity vs Switch. The switch has a lot more in common with phones than it does with other consoles or PC portables so IDK if we will see AMD SoCs with the right power profile for nintendo's needs.
Nintendo could use one of the new Samsung SoCs with RDNA2 graphics though. Those could be pretty good competition for Tegra X* chips.
I said that it would make more sense because they would open their platform to a lot more games, maybe it would cut a lot of dev work, and they'd might to it with competitive prices (like the Steam Deck has shown).
Now, Nintendo doesn't care much about that because they're into their own thing, they don't even see themselves as a tech or gaming company, the position themselves as an entertainment company.
Like, I think they see themselves closer to Disney than to Sony Computer Entertainment.
About the power profile, the switch came out with around 2h-3h battery life while gaming, depending on the game of course, I don't think it would be much of a problem.
Half or so of all Switch games are already made with Unity and I don't think Switch not being x86 is the reason most other games are not ported to Switch.
AMD has ARM licenses (IIRC, both architectural and for IP cores) and has shipped an Arm Cortex-A52 SoC in the past (Opteron A1100). If Nintendo really wanted, they might be able to order an Arm SoC from AMD directly. But yeah, going with Samsung would be far more practical.
Problem is, many existing Switch games use some nvidia-specific API (IIRC called "NVN" or whatever) instead of Vulkan.
Nintendo could use one of the new Samsung SoCs with RDNA2 graphics though. Those could be pretty good competition for Tegra X* chips.