Theres probably going to be like 5 thousand 8 year olds who want goku, even though he was in like 2 Nintendo games that are released on all consoles.
Now I think we're forgetting about the most important character to add, and that's my boy, Pokemon Trainer. Now I was thinking, that instead of having Kanto Starters, how about a region I feel gets almost no love, Hoenn. You give us Grovyle, Mudkip and Blaziken and we'll be good. Similar moveset to the old Pokemon trainer, but fresher.
Nah, some nice ideas there but I gotta go up to bat for my avatar pic Red!
I guess putting in a different Pokemon Trainer could work, but the thing is that Red is undeniably the most famous of the player characters of the series, both to fans of Pokemon and more casual gamers. Not only was Red the first player character choice, but he is the only one to reappear in subsequent generations as anything more than a passing quote. He's a respected and extremely powerful trainer in the series as a whole and not just one generation, and the fact that he appears at all in HGSS and BW2 is evidence of his fame and importance over the other player characters. This makes him special since besides some recurring trainer npcs like Cynthia, Cheren, and Steven there's not much that carries over between the different stories of Pokemon. Red is an important exception to this trend.
Also in popular culture at large, there's much more common and pop cultural knowledge about Red than any of his successors. Just type in google "Pokemon Trainer" then a space and Red is the fourth option that comes up! He has the most fanart and his battle theme is one of the most popular Pokemon songs. Also, besides being first one and the only player character to return, he is popular as well due to his similarities to Red from the anime (in the public eye, since they are far from the same person). The influence of Red on Ash is even apparent today, with Ash's hat in the current anime being a slightly modified version of Red's cap in FRLG and Brawl. Red is even the focus of lots of the satire comics and videos that people put out on Pokemon in general. So you have the most game appearances combined with the most cultural influence and recognizability, and it's clear at least to me that Red was chosen as the PT in Brawl for very good reasons.
That's not to say another trainer couldn't work, but I wouldn't want it at the cost of Red's spot for the reasons mentioned. Alas, the Pokemon Trainer concept won't appear again at least until the next games due to technical limitations so the discussion's moot really. And to be honest, I do prefer having 'Zard as his own fighter as it gives him more competitive viability via Flare Blitz. Also, it emphasizes Zard's proud nature and control over it's own destiny much like in the anime. And as much as I liked the unique play of Squirtle and Saur, it's kind of hard to justify giving them roster spots even over other pokemon like Lucario and Jiggs.
With all that said, I really want PT Red back in some capacity next game, and was sad when he didn't get an assist trophy, entering/victory animation for Zard, stage hazard, something to recognize a series veteran more than just a trophy. (Wolf did get those badass codecs ya know) If I had a choice, I'd either make him a playable character in his own right or just have him stand by and watch Zard or another pokemon fight like he did in Brawl. I guess if you really wanted you could change the trainer along with the costume color or something like that. But I did want to take the time to explain why Red was selected over the other trainers and why he remains the best potential human representative from the Pokemon universe.