How long before someone goes all the way by designing a completely custom motherboard and transplanting the key components onto it I wonder? It's been done for older systems but the BGA parts in the Wii would be a challenge.
As a longtime observer of the scene, no. Creating the smallest board possible while maintaining original game compatibility are about the only metrics to stick to. Usually this boils down to keeping the CPU, GPU, NAND (if exists like the Wii), and most times the RAM.
There are some nuts people out there like Redherring32 with his TinyTendo project that go above and beyond just putting the components on a new board. He designed the TinyTendo which requires cutting down the DIP-packaged CPU and PPU from the NES into a smaller package. These components were only produced for Nintendo systems of the era and are the special sauce for overall game compatiblity.
https://github.com/Redherring32/TinyTendo
It's not an apples-to-apples comparison, but the Wii's CPU was 32-bit, 729 MHz, and single-core, and its GPU was a 243 MHz SoC. I briefly checked the Samsung Galaxy S lineup as a point of comparison, and it seems like the second one (from 2011) already smoked those specs. As for space, even without doing any modifications to the phone it's already about half the volume of this mini-Wii.
Of course that's really just sidestepping the challenge of "how small can you get an actual Wii".
You could 100% make a smaller fake "wii" by emulating it with a smartphone SoC. Mid-range phones can emulate Wii and Gamecube at 2x resolution.
However, the intention of these miniature builds is to preserve the original device, just in tiny form. No emulation involved. As if it was an official product.