I thhink it was very clear that Kes wanted out of life. She wanted to learn and explore, that's why she joined the Voyager crew.
Despite that her full potential wasn't used, she was never "ruined" as a character (except for one disgusting attempt from those in charge which we, like the video clip said, don't have to go into now). In that case, all the other characters, except Seven, The Doctor and maybe Janeway were "ruined" because their full potential weren't used either.
I must also state that Kes did at least have some development. she came on board as nothing more than Neelix's girlfriend who he had rescued fro the Kazon. Then she developed into an important member of the crew. She became a nurse, she was the first one who regarde The Doctor as a sentient being, she grew fruit and vegetables in the Hydroponics bay and she did actually save the ship on some occasions.
Compare that to the development of Kim who remained "young ensign Kim", the whipping boy of the series. In fact, Kes had more development in three seasons than Kim had in seven seasons.
Note also that there werent much development of the other characters either after season 3. B'Elanna became superfluous after Seven arrived and they had to come up with the Paris/Torres relationship to show that both of them werestill on the ship. Tuvok was shoved in the background as well as Chakotay. Paris gradually lost all that had made him a great character in the first three seasons, Neelix was basically the same but Janeway was actually badly and inconsequently written in the later seasons.
That leaves only Seven, who became the new main character in season 4 and The Doctor who was the only character who really had some development during the whole series (soemtimes too much development like when he got that mobile emitter).
If they didn't have any plans for Kes, they surely didn't have any plans for the other characters either, except for Seven, The Doctor and maybe Janeway.