First; Mod it.
Get an external HDD, if possible, some old big hunk that has it's own power supply. Wii U infamously sucks as it delivers too little power to power most external HDD's, while it is also the official way of expanding it's storage. Nintendo just thought the basic type of external HDD around is them big buffalos with their power bricks. If you get a small one, that too should have optional power supply, or you need to yet a "Y-Cable" that draws additional power from another USB port, and that does the trick but uses one more USB slot. The console has two in the front though so it's 'k. This is also the reason why the official gamecube controller adapter for smash bros has two USB plugs, the second one is just additional power for the rumble, and switch for example can power the rumble from just one USB port. Silliest factoid of this is not that nintendo broke USB standards, they followed the USB standard to the T and delivers the amount USB2 should, everyone else just made breaking this standard a standard and usually output more power, including the Wii. You also want a fairly sized, yet still FAT32 formatted SD card. 32-128 gigs.
Wii U is quite powerful retroarch machine, but Aroma being best CFW, there is only unofficial, semi hidden builds of retroarch that work with it. But work it does. There is option of making injects to nintendo's official emulators but this eats easily the 300 title limit Wii U menu has, while also giving subpar, though child proof experience compared to community emus. This is also main reason you'd want a large SD card as it is best place to load ROM files from.
Once modded, not only is Wii U library accessible, through a
nifty downloader of sorts no less, but you can also instead of juggling multiple hard drives create injects of Wii games into the digital format sold on Wii U eShop, and launch Wii games from your main menu, installed on the same HDD as rest of your Wii U stuff. This though, requires creating the "packets" with your computer, with a tool, matching your console's and the game's region is more crucial than with a modded wii, or pre patching required, but you also can enable simulation of classic controller with Wii U gamepad. Not only that, but the "format" was also harnessed so you can inject GameCube games to the Wii U HDD, played through nintendon't, with some of the same additional features like GamePad support. Few games are incompatible (Like the internal web page for setting up PSO online in PSO ep 1 & 2). You also need nintendon't files and configuration on your SD card, so these do not run if you have no SD card at all.
There is
Pretendo community revival for the online game servers and miiverse, that supports few of the bigger games like Mario Kart 8.
My game recommendations? I will go with the weird, unique on Wii U stuff. May not be great but they are unique. Project Zero Maiden of the Black Water. The Wii U gamepad camera gameplay is very fun with this title. Very nice, if not the nicest use of the fun pad. Devil's Third, one of Itagaki's last games. It might not be the most wellest made game but man can it be entertaining with unique energy, and not on any other platform. Wii sports club is a nice upgrade to Wii Sports. If you get them MOTION PLUS wiimotes. The golf game for example makes you place the gamepad on floor as virtual golf ball you have to try to hit. It may even use the gamepad sensor bar for this. Yes, the black line with the camera in middle is actually a tiny wiimote sensor bar, usable in Wii Mode too. If you have family/friends, and do get them WIIMOTES WITH WII MOTION PLUS, Nintendo Land has tons of unique and fun social minigames that use the gamepad in fun asynchronous multiplayer ways. Zombi U, while ported on other platforms, also plays uniquely on Wii U as accessing your backpack (with the gamepad) is a real time thing that gives a semi augmented reality feel to the game like Project Zero does. Lastly, both Wind Waker HD and Twilight Princess HD are still stuck on Wii U, with gamepad inventory, motion controls and graphical improvements/changes (Wind Waker's look splits opinions, mine included) that make Wii U version play the best, and also add Hero mode to the two titles. While it's main game was disliked, Star Fox Guard is quite liked, small game. Puyo Puyo Tetris has an English patch.
Anything you want to ask or clarify, I will try my best. Had my Wii U for years and it is still cool what stuff it can have on it, especially with Wii+GameCube injects. Heck you can even play PSO EP1+2 online with it, if you fiddle around and launch the game image from SD card instead, one more point for a larger SD card.
Originally, you had to have a large SD card for the installable Wii U games. You might still want one as it is possible to back up, and then install to your HDD/Internal storage your game discs, but this requires spacious SD card. The nifty downloader I mentioned earlier defeats the need for... the other ways of getting games.
