You can't overwrite the Wii U firmware with custom firmware, but outside of that you can do everything else. Even then they have a work around for custom firmware that you can run off of the SD card though.

You could download games straight from Nintendo's servers through some back door and install them straight to internal memory or an external drive and run them like normal. FOR FREE. If you can get it from the eStore then you can get it.

I'm sure the independent games are cool too (I guess).

Funnily enough, Twilight Princess HD eats up half of 32 GBs. It must be a staple of the Zelda series, like the Master Sword.

If you tryna play Bayonetta, for the cost of a 32 GB SD card (I started with a 8 GB and soon realized some games were at least 10 GBs) and a SD card Reader, you could set your Wii U up to play that and any other game in 2 hours from scratch.

I fucked around and copped a pro controller. Now it almost gets as much burn as my PS4 did. That's until I cop another PS4 at least.