There are a number of character I think should have had more dev.
Cilan - He literally had little to no development at all. I think Pansage should have evolved and I think Cilan could have easily caught a few more pokemon. He really should have caught a pokemon when they went to East Unova with older pokemon.
Tracey - I did love Tracey, but sadly he was only there because he was supposed to replace Brock. I feel Tracey would have been a better character. He really didn't have time to get much development.
Max - Max never changed in the end. It was just annoying. I really don't understand why they added him randomly like they did, unless they eventually plan to make a Series for Bonnie and Max.
Iris - I don't think Iris had enough. Both Iris and Cilan completely failed to draw the attention of a lot of the fanbase. Iris didn't grow. She continued to accuse Ash of being a little kid. Yet he ended up with more development even though he was reverted by a lot. Axew should have fully evolved. She should have caught Gible during the main saga. She should have started with at least a Dragonair and not a freaking Dragonite. All the damn dragon type they added and she caught a freaking Emolga which really annoyed me. She should have probably gotten a Dieno instead of a Dragonite.
Brock - Out of all the time Brock was in the anime, he should have gotten a load more development. He literally did not change a bit. Most of his development came in the episode that were in Pewter City. He didn't change until the very end and then he suddenly wants to become a Pokemon Doctor and not a Breeder randomly, with no hints of him changing until the very end. And I think that was just so they could kick him off the show as a main character.