Why do you think they created Trip instead of using Cheren as Ash's main BW rival? Both Cheren and Trip both have similar goals and you could really change them up and it wouldn't be much of a difference.
I can see why Paul was created. Since Barry is your main rival in the games and was more comic relief Ash needed a serious rival in DP. But Cheren is your main rival in BW and he might not be a jerk like Blue and Silver, but he was still serious while Bianca was more comic relief. Why didn't they just use Cheren? Why use Trip? If they wanted Cheren to be a Gary/Paul type jerk then they could have made him that way in the anime.