Reddas was supposed to be Balthier. Or, Reddas' actions and story were supposed to be Balthier's.
We already know Cid made Balthier a Judge by the age of 16 (the Ultimania does state Balthier was clever and good enough to be one, so it wasn't just because of his father - although this could have been changed later, Balthier originally written as to have been used by his father as a tool) and that at some point, Balthier ran, stealing the prototype airship The Strahl, living life as a sky pirate.
Reddas was also a Judge (I like calling this game "Final Fantasy XII - Everybody used to be a Judge"), given the nethicite by Cid, Balthier's father, to see what it would do to Nabudis, thus destroying the entire city, becoming a sky pirate after that, living his life in Whatever Mode, like Balthier. (The flashback with Ondore shows he's drinking heavily.)
I also found it a bit of a cheap way out that they would introduce Reddas then have him commit this huge sacrifice. It's a brilliant moment, but imagine how much more emotional it would have been if it had been Balthier. He would have said his goodbyes to Fran, finally deciding to face his past and clean up the mess and pay for what he did to Nabudis, then picked up Ashe's sword and went for the Sun Cryst. The series of events even fits perfectly; Reddas comes up to Ashe from behind and takes the sword moments after the Fran and Balthier scene. "Give me this sword and let me finish this" etc.
The whole "Balthier saves us from Bahamut" thing seems to be a scene written in to fit the "Balther must make a sacrifice" thing. Sure he saves them, but it seems like a cheap way out. Fran is even sorta unconscious in both of them, so they're even kinda similar. The original version of the Bahamut scene has Balthier say something to Fran like "you sure make a lot of work for me don't you", which could also fit for the Sun Cryst scene ("omg I have to carry you out of this thing?") (The translated version of the Bahamut scene makes it seem like Balthier is asking Fran to give power to the Glossair rings, while in fact he's stating that they already have power - and when she doesn't respond (read: give him credit), he says the "you sure make a lot of work for me" thing, implying he now has to carry her out, and not "do I have to do everything myself around here".)
So I think that in the earlier storyline, the one where Vaan didn't exist and the story was more focused on Basch -- Balthier was supposed to self sacrifice, repenting for both his father and his own actions, and then he was supposed to die. He was probably even supposed to be older too, closer to Basch's age - and Reddas is 33.
I also think the "2 years later" was supposed to be "10 years later", aging both Ashe, Balthier and Basch, but they had to cut it down to 2 to keep Vaan young, and make more sense to the Reks/Vaan storyline (them being close brothers, which they probably wouldn't as much if Reks was 10 years or so older than Vaan). Also 10 years is too long a timespan for younger players to have a grasp of.