They should have launched an evolution of the Wii instead of an evolution of the GameCube; that's what the Wii U was: A GameCube successor with the Wii brandname slapped on it.
An evolution of the Wii would have had improved motion controllers as its standard controller. Better motion sensing, no cord between controllers, built-in batteries in each of the two controllers; basically, the very same things that Switch has in its standard controller. Naturally, all flagship games of the Wii should have seen sequels as well; Nintendo didn't even bother to make Wii Sports 3 for the Wii U. Pack in an updated Classic Controller with each console (price wouldn't be an issue because the Wii Classic Controller was sold for only $20), that way neither developers nor consumers would have had to complain about a lack of classic controls in games; consequently, let games have multiple controller options by default, unless it really doesn't make much sense to do so.
Virtual Console would have started over again because Nintendo didn't think ahead with the Wii, but just like the Wii U didn't require repurchases, the Wii 2 wouldn't have either; only if people wanted new features like quick saves, button mapping and cleaner HD graphics.
Indie developers were better catered to with the Wii U already, so the Wii 2 would have been good to go anyway. Significantly better hardware sales due to a straight-forward concept (evolution of the Wii) would have attracted more indie developers to the console. The AAA third parties wouldn't have liked the Wii 2, but they don't like what Nintendo does, no matter what Nintendo does. That's why it isn't worth thinking about it.
The Wii U wasn't a necessary evil to arrive at Switch. A hypothetical Wii 2 and DS2 (because the 3DS's 3D was a bad business decision as well) would have required longer development times for games all the same, so the consolidation of software development for home consoles and handheld consoles would have almost certainly happened anyway, especially because another factor (Sony's exit from the handheld market) also played a big part in Nintendo's decision to do that for Switch. There was no realistic way that Nintendo's first software output would have reached the same amount of games as the Wii and DS had, even if Nintendo had gotten both the home console and handheld hardware right. But instead of selling ~90m units, they would have easily sold more than 150m units of hardware due to more affordable launch prices. Wii 2 could have launched at $250, DS2 at $200; both without any concessions to the processing power the actual Wii U and 3DS had.