In terms of characters, this is a very, very, early idea right now, but my thinking at the moment is that in the second movie, the droid (HX-81 is the rough version of his name) actually betrays the cause and alligns with the Mandalorians, strictly because he was so cynical about the Republic's chances in the war, that he thinks its safer for him (and his allies) to switch sides. This action also likely leads to the death of Darelda [the alien main character]. I know the immediate reaction will be that having a turn in the second film diminishes Anakin's fall in the third, but I think it could work, because it accomplishes a lot of things.
First, it sells how badly the Clone War is going when one the critical characters actual betrays the cause, simply because the odds are looking that hopeless. Second, its part of a larger demonstration of the dangers of pessimism that are big throughout (my concept of) the prequels, something that also plays heavily an Anakin's story later on. Third, it does help to explain as to why such characters would not want to be brought up. Kenobi and Vader likely have very painful memories and have wanted to move on from both of them.
Fourth, that it differentiates how different these characters and this group are from those in the original trilogy; in fact part of my thinking has been to follow through on what if scenarios of the original films: What if the betrayal by a trusted friend (Lando and Han) led to much worse consequences than it did? How would a group of heroes react to losing on of their own? What if the droid is more than comic relief? I know some would point to it as being pretty dark, but I think there is some story potential in seeing the traditional "Star Wars heroes" in a scenario where everything just went terribly wrong.
Finally, and I think this is the biggest aspect, its a critical build to Anakin's fall from grace. Skywalker is a young guy by the second movie who's still managing to hold his optimism pretty well, in spite of the war, and even though he shows great anger and rage at the betrayal of 81 he does not turn, because he still believes in the cause. Indeed, he vows to bring 81 to justice out of disgust that the droid could turn on his own allies all out of his own lack of faith in the Republic and his friends.
But the sad irony of it all, is that Anakin does a turn even worse than that, largely because the tolls of the war as well as seemingly losing his wife and child drives him to a much worse place than even 81 was. While the droid may have been a cynical pragmatist doing a preemptive move decided by reason, Skywalker ultimately gives into the dark side because of sheer despair. He's lost so much, and simply can't see any way but Emperor's ways to make things better. Losing two friends (one to betrayal, one to death) may have not turned Anakin to the Dark Side, but it pushed him a hell of a lot closer to it.
