On why they didn't use Hugh. Because of the Team Plasma angle possibly that seems to play a larg part in his role in the games. On why they didn't use Kyouhei, because he looks like a pizza delivery guy gone wrong. XD
Why should they? There's really nothing about those characters that would have made them especially interesting as anime rivals. In the games, they're fine, but the anime's story has never followed the game's too closely to begin with. Besides, having Ash's rivals be close in age to him works better.
And they've never used a PC as a rival, so I don't know why you expected them to do that.
Well, they couldn't use Kyouhei because Ash is already pretty much a stand-in for the male protagonist of the game, and as for Hugh, I agree with @Hellion that Hugh cannot be used because of how Team Plasma is involved in his characters backstory, and because the Plasma Embargo is still in effect as far as we know, Hugh cannot make an appearance in the series unless the writers attempt to write around that part of his backstory (which is pretty much impossible).
Still, why not Cheren, while Bianca has appeared various times in the anime, Cheren has had not a single mention or appearance, even in cameo form. Is Cheren's character not interesting to the writers or something, is that why they don't use him? I know he's a Gym Leader in the games now, but that still doesn't give the writers of the anime an excuse to not use him.
So for now, we get Kotetsu, whom, in my opinion, doesn't appear that he will be an interesting character, even with a Riolu and a Samurott in his team, he doesn't appear to be all that interesting of a character, but of course, only time will tell if he will be truly significant to the series.
idk they somehow luv Trip
Hugh should be related to a Team Plasma story & Kyouhei - They don't use male protagonists in the anime as main's or rival's.
I'm glad Mei isn't brought up here because she would pull a Moon Prism Pokémon.
I was under the assumption that they didn't use any BW2 game characters because those characters had the animated shorts to promote the games, and because of the time difference in that BW2 takes place two years after BW. Especially with Bianca still a rival and not a lab assistant as she is in those games, it might have confused some people if they had used them as a rival, when in the games, they received their first Pokemon from her. Different verses, I know, but it's probably easier this way to keep all their canons straight or something~
probily due to them being Promo characters and possibly movie openers such as SIlver, Brendan, lucas, ect
Maybe Cheren could appear in his Gym Leader role? I know Ash already has his eight badges, but that didn't stop Marlon appearing.