Huh, so I'll give scores then arguments since the concepts have, at the core, similar issues and benefits
Master Hand
Chance - 1%
Want - 0%
Playable Boss Mode
Chance - 5%
Want - 10%
What I like about these two ideas is that they bring forth both uniqueness and the whole concept of asymmetrical gameplay that's supposed to be the core of Wii U gameplay since Nintendoland.
For Master Hand, I'd say it's rather unlikely given the fact balancing it could potentially be a nightmare, and with this Smash it seems to be the case that Sakurai is very conscious of the fact each potential character has a very strong fanbase that he doesn't want to disappoint. He's already given us WFT out of left field, at most I'd imagine he'd give us perhaps one more random character (perhaps a retro or new franchise) and the rest should cater to fans. Master Hand, while acknowledging the legacy of Smash Brothers, seems like a selfish decision since it's going to be in the game anyway, and having it take up a player spot will probably detract from it's boss appearance and remove the chance of another Nintendo icon to join. This also explains my 0% want.
The only way Master Hand works is as a playable boss controlled through the gamepad. So we move to that. Now, personally, I've also given thought to the fact that both versions will have a second screen with touch controls, which would make it easy for such shenanigans as controlling the Master Hand with the stylus to mess with other players, in a boss mode or outside as a stage gimmick or separate multiplayer mode. It's a really solid idea, along with many of the other gamepad ideas people have come up with ... heck, I personally feel just having someone with the gamepad control the spawn location and type of items would be a brilliant way to include non-core gamers, and it would be fun to have the gamepad control the trajectory of the smash ball. Nevertheless, I recall seeing in an interview somewhere, can't recall which one, that Sakurai seems keen on not having gamepad gimmicks, and a playable bosses mode would, imo, just not work without taking advantage of that. So I'll bring chance down because of that.
As for want, I bring it down because I'd rather Sakurai and the team focus on balancing the game as best as possible, because in the long run that's what we'll be playing over and over again the most. I'll sit to play Smash in the sense I'll go online to practice my Palutena skills (cross fingers), I won't be playing it so much to go on bosses mode, aside from the occasional party I suppose. I give 10% instead of 0% since I do think it'd be nice to have a non-gamer friend/significant other control an overpowered character to help you train.
Predictions
Stork - 0.27%
Zelda newcomer - 54%
Nominations
x5 Magolor