I'd like it if it were for both, and you could play together across the systems. Then everyone would have their own screen to explore.

And I'd hope that a game like that would rather improve the sales of the third party games, through increasing the audience.
I don't feel like the market for (A)RPGs on the 3ds is overflooded (at least in the west), so people should have enough time for both Nintendo and 3rd party games.

I don't think Nintendo would ever create a true-to-the-genre simply because they love the sense of action/adventure (other than platforming). The closest they may have to doing something like that would be Faxanadu since it does not quite incorporate a number-leveling system.

I just have this feeling that Nintendo has the notion that experience, fun, gameplay, etc is not simply measured in numbers.

They seem to treat every aspect of their philosophy this way.

Nintendo really doesn't try to rigidly adhere to a specific game genre when designing a game anyway, they just kinda make each game work however they feel is appropriate. (We also have a very flawed definition for the word "genre" in this industry anyway, generally it is supposed to define narrative tropes and structure.)
I would not be opposed this Diablo-ish action dungeon crawler the OP suggested, however they would have to give it a unique flair to make it appeal to me.

Okay, so I'm glad I popped back into here since there are actually some people who are on topic, so here goes :D

Call me greedy, but I want more than bits and pieces! As far as it being kind of niche, sure I'd kind of agree to that, but back in the day when you went over to a buddys house, you were playing a fighting game, a racing game, a sports game, or one of the genere I'm looking for.  The Wii U itself is sort of niche if you think about it. Its got your platformer fix(traditional and non), your retro fix(VC), your kart racer fix, your smash fix, and a few other niche generes nowadays. I think it might fit right in. I'm not saying it would be a system seller, or even a huge seller... just fun! :D

For sure, a Diablo 3 port could definitely work and would actually convince me to buy the game for a third time:P Getting the controls right is obviously a must, but some of the options you would have with the game pad could be truly great. On the fly inventory management, mapping, social, character management, etc... Seems like a perfect piece of hardware for these games.

Brilliant idea! Honestly I don't know why I didn't think of that sooner. Maybe not an actual Mana game since thats owned by Square Enix and the little faith I have left in them is due to the Bravely IP. Something with the graphical charm and great gameplay of the Mana games with Nintendo's unique spin could be fantastic.(I still want couch co-op tho, the mana games had that :D)

I fully agree with this statement. A great fit to help round out the Wii U's existing library. Nintendo has options, resources, and the connections to make this happen, even if its not super popular.

How does Diablo 3 control on a console? I 've only played it on PC so far, and am really curious how the gameplay transitions from mouse+keyboard to Controller.

If the Wii U could handle it, playing Diablo 3 with two players should be quite possible, as you have 2 screens.

Plays great on PS3, at least the vanilla version of the game. Not too sure if Reaper of Souls came out for 7th gen or not, but there was no lag I came across durring 4 player mobbing... sooo I mean I don't see why the Wii U couldn't handle it in terms of hardware.

As far as the controls go with a controller... they're excellent imo. Inventory isn't tetris style like on PC, its just like 60 slots for you to carry stuff. The abilities are easy to use. Easy to dodge and move around... honestly I perfer the console controls to the PC.

I do think that the Wii U can handle the game on one screen, but I am doubtful it can handle local 2 player action, with one player on the Gamepad running around Tristram and the other one on the TV being in a Rift. Single player I'd imagine having the Gamepad for inventory purposes or as a map overview or for selecting skills would be convenient.

Action RPG wise I'd be pretty happy if Square Enix could bring Secret of Mana, Secret of Evermore and Secret of Mana 2 to the Virtual Console, and while they are at it also bring FF6 and Chrono Trigger to Europe. I'd even except a higher price tag for Secret of Mana 2, as it would have to be translated.