This post was originally intended to be a reply to AtomicB's thread about Geno in Brawl, but after gaining a little bit of momentum, I decided this post could function as its own thread, and would appreciate the feedback.
I am well aware of this Geno's popularity, both from Sakurai's poll and from the buzz that's been generated from SmashBoards. And although I agree [with AtomicB] that Geno makes for a potentially unique addition to Brawl, I do not believe he "belongs" in this roster for 2 big reasons. (Important: although the first part of this post concentrates on my reasons why Geno wouldn't belong in the roster, the second part of my post outlines a plausible way of making him an important part of the game as well as a playable character)
Part 1 - Against
1. Star Power: Geno made his first appearance as one of the playable characters in Super Mario RPG. Since then, he has only made one cameo appearance in Mario & Luigi: Superstar Saga. Geno simply doesn't have the star power to be featured in the Brawl's playable roster. While one can make the argument that there are characters like Ice Climbers who have only been featured in one game prior to the creation of Smash Bros, the fact that they were the titular characters in a game created solely by Nintendo was enough to put them over the edge.
As a matter of fact, a number of roster characters have had minimal game time. Pit has only been in two games (not counting cameo appearances) while newcomer Olimar has only been featured in two games as well. What sets these characters apart from Geno is the fact that these characters were the sole main characters of the games they starred in. They single handedly represent their respective Nintendo franchises and as a result, carry the star power needed to obtain a position on the roster.
2. Third Party Power - What sets Geno apart from other confirmed third party characters as well as other potential third party participants is his lack of third party power. Solid Snake not only represents the Metal Gear franchise, but also Konami as a whole. Likewise, Sonic not only represents the Sonic franchise, but also the entire Sega corporation. Geno, whose rights are owned by Square-Enix, just doesn't have the power to represent Square as a corporation.
I agree that it Square would probably have no problem lending Geno to the creative minds behind the Smash franchise. In fact, if Nintendo really pushed for it, I don't see why Square would object to selling all rights to the character to the Nintendo corporation as I doubt Square has any long term plans for Geno.
If a character was going to come out of Square-Enix, it would be a character that would be able to bear the weight of representing a multi-million dollar company. Other potential third party characters (Megaman comes to mind) have much more third party power than Geno has.
Part 2 - Geno in Brawl
I'm not saying that Geno won't happen, especially since he scored so high in Sakurai's poll and dozens of predicted rosters have included him in the list. If he were to be included, Sakurai would have to do it in a way so that Geno would obtain Star Power by being in Brawl. Let me explain this thought:
The Subspace Emissary puts our heroes up against some of the biggest villains in Nintendo history. However, although characters like Bowser and Dedede are clearly up to no good, we know there is a driving force behind all the events that occur in this game. The Subspace Army, led by the Ancient Minister is the main antagonist that ties together all the bosses and bad guys we face during the game.
Likewise, it would make sense for the Subspace Emissary to have a main protagonist who could tie together all the good guys we come across. Geno happens to fit the bill, especially since his character is a spirit sent from the high heavens to maintain good in the world.
With the power to bring any inanimate object to life, his role in this game does not need to be limited by the doll that appears predominantly in Super Mario RPG. Although this would most likely be his main form, Geno could breathe life into many surrounding objects (rocks, broken Subspace machines) to guide the player through the game.
This way, Geno would be launched from Mario's one-time sidekick to main protagonist of one of Nintendo's biggest games of 2008. Not to mention, it would make thousands of fans happy to see their most wanted character represented in a new, original way.