Will they ruin it? Ha ha ha ha. How can they? The best thing that could happen is to give some anime only viewers is some development about the characters sooner than later. Don't you like when they say: "Men, this character is so boring. There's nothing interesting about him/her"
Well say Momo. While she doesn't have much screen time in development in the sport festival arc, she does have an arc where she does develop in the future. Giving her development now wouldn't make sense.
But to take the that even further, say Bones decides to give development to a class B character that doesn't have any screen in this arc. However, Horikoshi may have plans for said character in the manga that's completely different from what Bones plans to do with the character in anime. So when Bones finally reaches the point where the character SHOULD get a lot of screen time it'll leave a inconsistency in the story.
DBZ anime faces this same issue and there's some inconsistencies in the anime due to the anime staff adding developments to the story.
Sure, they could approach Horikoshi about characters, but Horikoshi has enough work on his schedule with the weekly manga alone. And it's a lot of unnecessary extra fuss when they already have a perfectly good story arc to adapt on its own