They’ve done this at least 4 times in the past already (see post 62). Don’t know why you think this is so unlikely.
I don't know about the Gameboy Color or N64 Expansion Pak...
Gameboy Color wasn't marketed as an incremental hardware revision but as a new machine with backwards compatibility, and the Expansion Pak was even more akin to an accessory, like a light gun, than a 1.5 hardware upgrade.
I honestly forgot about the DSi.
DSi is an interesting example because it's more of a side-grade: you trade a camera for the GBA slot, and the camera nets you a handful exclusive games.
Then you have access to the eShop, which is smaller, cheaper games - not exactly a 'Pro' model.
The DSi fits more with my speculation, I think you're looking at some basic QoL (bezel, kickstand, clamshell), but there's no analogy to the eShop or exclusive games.
The only thing I could possibly imagine is raising the performance ceiling in handheld, but again, unless some leap in battery tech happened in the last few years, I doubt it.
If they're juicing the docked mode, they could do it with a dock - then we're looking at the Famicom Disk System and 64DD as the precedence.
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I think it's unlikely because it doesn't make sense from Nintendo's perspective. Switch is super successful, and you don't make it more successful by fundamentally moving away from it did so brilliantly: landing every hardware compromise right on the line of 'good enough'.
Even if you're taking a historical perspective, the present circumstances are dramatically different.
They have zero competition right in the hardware space. That's unprecedented. I don't see anything past a nifty QoL revision (a 4/5G console with a smaller bezel and decent kickstand is the the craziest thing I'd hedge my bet on) happening before Sony and Microsoft show their plays for portable screens.