I would rather have a better story than more characters. If Duckys father contributed to the story then I'd be in favor of him appearing, but he really has no purpose in the movies. They don't draw a useless character if they don't have to. Have you noticed that Littlefoot is the only character in the whole story that has 2 guardians that are regularly seen? I guess it's cuz Littlefoot always turns to them for advice and it's easier to have a 3 character dialog when advice is being given. It shows 2 pov's. Besides, if they spent time on other characters, the movie would take longer to make and it would take away screentime from other characters and possibly damage the quality of the story.
Of course, they do animate random, minor characters during certain moments when need be, even though these characters contribute absolutely nothing to the story.
I personally see nothing wrong with introducing new characters, as focusing on just the original five could get tedious. It's nice to have something new every once in a while.
However, the number of characters in each movie would have to be kept in check. When too many main characters have to share the same spotlight, things don't usually turn out well. Neither character would get a decent amount of spotlight. One such anime that seemed to suffer from this, unless I'm mistaken, was Transformers: Robots in Disguise.
And there's also animation difficulty. If there are too many characters in one scene, it can make the animation harder to pull off. I wouldn't say it's impossible, but it would be difficult and would definitely take longer.