Samba shares?
Eliminates the need to buy another storage device.
FTP?
Could work as long as you're at a hotspot and your home network is running.
Serious question
You gonna carry around an HDD whenever you game on the go?
Honestly, the nand and/or an SD are large enough to hold the 1 game you can play at a time.
Yes USB hdd support is going to happen, and that will be pretty cool for home use (not on the go PuNKeMoN lol, that'd be kind of ridiculous unless it's complete setup is small and not too bulky but still no lol home use is better for usb hdd support)
However along with hdd support, you kind of sideswipped the idea I was going to suggest but not quite. Not samba or ftp, but how about implementing in the custom firmware like rajkosto elaborated on what's required to do...
A WiFi backup loader (similar to hdd backup loader), where whenever the system needs to read the game data (or next block or chunk of game data) for the game, it thinks it's reading it off the game card, but custom code hijacks that execution (probably through the hypervisor layer) and instead reads the data from your local computer or phone or tablet or whatever over WiFi (launch a hotspot on your phone on the go, or connected to your home network read from an hdd on your home computer) and transfers it to the switch where it is then loaded by the system and the game plays. The server written for computers phones and tablets and run on any practically any os where you have your owned game backups stored, and the client running natively on the switch with the hypervisor added wifi loading support!

For convenience it can even have some thing where it automatically finds these servers running on the same network your on (probably would want some kind of authentication so if on a more public network for example only you can use it) so the inconvenience of manually specifying the local server ip will not have to be done most of the time.
It could be done! And would be still pretty fast (on a good network mind you) that you might not even notice it's running off of wifi rather than game card! This way you wouldn't have to tether a usb hdd to the switch or dock or whatever but you would still get basically the same functionality!
EDIT:
(currently it only supports HID class and the very specific wii/wii-u network adapter chipsets)
And if it already supports ethernet (I'm assuming through the very specific wii/wii-u network adapters mentioned here) then if it's implemented it would already work the same exact way it would just be wired connection instead of WiFi. Also if your home computer hosting the server is also ethernet connected, then it we be even faster loading and you for sure won't notice!
Who thinks it's a good idea? I think so!
