There was potential for Optimus to have character development in Transformers Prime but the writers seemed to isolate his character and make him cold and flat as a consequence.

If they imbued some of the elements from G1 Optimus - the desire to be not only a leader but comrade and friends with your soldiers; the curiosity to understand humanity ... especially from a child's eyes. In fact, if they had taken the opportunity for Optimus to reflect on how distant he had become due to the burdens of leadership and time... the Orion Pax storyline would have led nicely into exploring that.
Optimus's fatal flaw (excluding movies) is always the unwillingness to take ANY life and I would have loved to see the show confront that more; give him more personal interaction where we get to see how he FEELS and reacts and grows as a character because of that. There were ample situations they could have done this but chose not to.

As much as I dislike RID2015 there was a hint of Optimus getting a bit more personality/humanity/reality check when he was with Windblade (when he wasn't giving ridiculously pompous speeches). I would have liked to see them continue along that path of the two characters discovering themselves thanks to each others flaws and insights. I see massive potential with their pairing to create a father-daughter/teacher-mentor relationship that I can see kids liking. My daughter LOVED seeing Windblade and Optimus working together.

Ultimately Megatron will ALWAYS win because of Optimus unwilling to sacrifice innocent lives but maybe that could change too if they actually did try to give Optimus a character arc where he has to sacrifice something or someone.

Interesting to note as a complete side: At Cybertroncon they had a video that showed the evolution of Transformers cartoons over time. When Transformers Prime came on the Chinese audience cheered loudly and there were calls of "bring Transformers Prime back!" much to the surprise of the head of Transformers media so the show definitely had some popularity behind it.
There were also lesser cheers for G1, Beast Wars and Animated.

Personally, if I could get Transformers Prime animation with Beast Wars storylines and G1 personalities that would be my idea of a great Transformers show.