even so, I shall venture to say that Gardevoir is not near so important as Meowth, nor does she have near the recognizability or fan base. While she may be more deserving than your run of the mill pokemon, she isn't stand-out enough to merit her inclusion in my opinion, and the opinion of many others.
With all the pokemon already included, a pokemon would have to REALLY stand out in most of the afformentioned categories to have even a chance of entering.
Team Rocket's Meowth is really the only pokemon who fits the bill, though I guess lucario should at least have a chance because of all his support. But honestly those are the only two that have any shot at all, and ven those two have an uphill battle with PT's inclusion
Hmmm, I dunno about the fanbase... from what I've seen, it's rather large. And considering that she got that big role in that game shows that she's being recognized by Nintendo and everyone as something special.
Whatever the case, she managed to jump into the category of Pokemon actually worth considering, who are not "random" ( like, say, Blaziken ). And no, being legendary doesn't get you onto that list. That stopped being special somewhere around generation 3.
From that point on, Pokemon are chosen just like other characters from other games... that means the deciding factors are recognizability, personality, fan-service, uniqueness, and, basically, Sakurai has to think they are worth something.
And considering that Gardevoir's the only Pokemon to have been addressed on that infamous poll thing, I'd say he does see something in her.
I think the Pokemon who are on that "list of notable Pokemon" that I spoke of are, besides those that are already in Melee:
Mew
Lucario
Deoxys
Meowth
Munchlax
Gardevoir
I probably forgot a few, but it's still not a big list. Note that I excluded those who just don't work, like Lugia.
As for Meowth... I think he had his chance. His fanbase isn't getting any bigger, and he wouldn't offer incredibly much in terms of uniqueness or personality.
Arguably, neither would Gardevoir.
We'll just have to wait and see.