Adding more RAM or even having more RAM when Wii U was released wouldn't help 3rd party support. Heck, the last gen ps360 are still getting many of the big releases and they have much less RAM.
Expansion of memory or other devices (ie: 32x) worked pretty easily because they connected directly to the main board through the cartridge slot. As Grahamf noted, the n64 had a specific expansion slot already.
There are ways to use sd cards for RAM but that wouldn't really be worthwhile.
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