I'm not sure that I like to call Mewtwo a Gen 6 rep because I don't think Pokemon will be truly given characters on a Generation by Generation basis. It should just be who is popular and who is relevant to what Game Freak has going on for the series at the moment.
If a Pokemon is chosen by looking into the movies, like Lucario was, it will basically be between Genesect and Mewtwo. My money is on Mewtwo based on his popularity combined with this. The only way I see Genesect and not Mewtwo is if Sakurai decides that Mewtwo had his chance and wants to incorporate new faces into the series.
Also, to spark conversation (i'm really hoping I don't start arguments with this, but here we go), there are several arguments based on the fact that recency only gets a character noticed, blah blah blah etc... But when you really look at Brawl's roster, and think about it, old characters from particular series may not have much of a chance if they have nothing to merit their inclusion. While we have Mr. Game and Watch, R.O.B, and the retros, they are in under specific criteria. We have never seen a random older character from an established series, unless we are referring to what I refer to as "Smash Mascots", such as Marth for Fire Emblem. Here are some examples and some partial reasoning imo...
Skull Kid- Popular, and from a popular Nintendo Title. However, at the current moment, there is absolutely nothing to merit his inclusion. He is not currently important to Zelda's plot, and would be a very random addition. What may boost his chances, however, would be a Majora's Mask remake.
Lyn- Very popular. Some regard her as the face/ beginning of Western Fire Emblem. However, isn't Ike the "Western Lord"? Lyn has not appeared in a starring role, or at least a role that has her stand out from the rest of the cast. This goes for several other Fire Emblem lords, like Hector, Ephraim, Sigurd, and unfortunately Roy (however, Roy has the advantage of already appearing in Smash Bros., time will tell if this will bring him back). Without some sort of focus on Lyn (or the other lords), or a remake of the games that these lords came from, I personally find it hard to believe that Sakurai would choose one of them over another in the same situation. However, Shin Megami Tensei X Fire Emblem may change this, but so far, Chrom has appeared in recent advertisements. Time will tell with this as well.
These are the first two thoughts i've been able to type out (my thought process just kind of spills out sometimes). However, my point still stands. I'm not trying to say that only recent characters get in. I'd probably even argue against that. I'm just saying that we haven't seen an older character without an important recent appearance get in from an established series, and i'm not entirely convinced we will either.
EDIT: I'd also like to add that while I am going to assume it will be on a monthly basis, my excitement for the character reveals makes me hope we get one sooner than we expect. However I must force myself to realize that my excitement does not mean we will get anything at a certain time. I just want a newcomer...