The question is: can it be done in a way that is profitable? Every wiiU comes with a gamepad, so as long as wiiU games can only handle one gamepad, the demand is pretty much nil (remember: broken gamepads fall under the warranty).
That demand will rise once games not only allow for multiplayer, but ALSO a multiplayer where it pays of to have two screens**. A head-to-head FPS, for example (though it'll take some serious processing power). Until then, I'm pretty sure people will, at best, buy wiimotes for their friends and guests.
There are numerous reasons somebody would want a cheap new gamepad, so the demand would hardly be nil. Things such as kids being hard/careless with toys, accidents happening( I'm sure there are numerous people here who have lost a phone in the water, by dropping it, etc), you name a scenario. Stuff such as this usually isn't covered under warranties unless you have a protection plan.
I already mentioned that gamepads under warranty period will be replaced. No need to buy a cheap gamepad if you can have the current one being replaced.
But I should have been more clear on the demand. It's indeed not nill...but it's lower than it should be. For two reasons:
1. as long as only one gamepad can be used at the same time (or, more precisely: as long as there's no 'killer app' that uses two gamepads), I don't think we'll see even an attempt at a third party gamepad. Likewise: people do tend to buy wiimotes or other controllers to replace the first ones...but the majority buys them partially because they have friends they want to play with.
2. there already IS a cheap gamepad replacement. In fact, I already have four of them lying around in my room: wiimotes. Sure, they have no screen...but how many games are actually going to use them? That's the point I was trying to make. I bet a lot of parents will tell their children something like "if you break that screen thingy, you'll have to use THIS thing instead".
The thing is: with all the technology in the controller, it'll be hard to make a profit as a third party developer. Take that JXD S7100 (image: see Kingvamp's post). That thing sells at 140 dollars. I doubt things can get much cheaper without making losses (which is straight out bad business for third party developers: they simply cannot afford that). But...for that same price, you have four wiimotes. And probably at least two of those pro gamepad controllers. Both of which can be made quite a bit more sturdy than the gamepad.
Fuck...the more I talk about this, the less likely I think it'll be there ever will be one. An
exclusive wiiU gamepad controller, that is. An android app that lets you emulate one, on the other hand, will become pretty popular.