Hm, it'd be different to have two people competing for Gym badges at once. Heh, Ash and whoever it is could take turns on seeing who challenged the Gym Leader first. The problem is that, well, having two important Gym battles in a row kind of takes away from the tension, or might drag out things even more since they'd had to keep visiting that Gym until Ash and the other person both get badges. Unless Ash took a backseat to the new charater, but... nah, like that's ever going to happen.
I know Brock doesn't do much, except be comic relief and occasionally provide random knowledge, but still, even if they replaced him, would his replacement be any better? He (or even she) might have even less to do than Brock. They tried that with Tracey for a bit, but Tracey wasn't all that different from Brock either, at least in terms of fulfilling the "smarter, slightly older" person of the group, then they brought Brock back. If they ever did replace Brock (which may or may not be a good thing), people would probably complain if he either took time away from Ash (I've heard that complaint aimed at Dawn) or if he didn't do enough.
I've liked the contests of the last two generations in that at least it gave the girl something to do rather than just walk around carrying an evil egg. Doing it once with May was different, if Dawn wasn't so likable, doing it a second time would've been more annoying, but not sure if they should do it a third time.
I thought one of the best things about Sinnoh was its mini-arcs, like the Summer School or the various small competitions they did here and there (Tag Battle, Wallace Cup, etc.), which made it seem less like filler. The best is if they had a bigger overarching plot then "on to the next Gym/Contest!" but that would probably be too complicated for this anime.