As great as Xigbar and Saix are I can't see them carry an saga, also it's probably better to start the next saga with a somewhat clean slate when it comes to the villain. I can see this two as minor recurring villains, but nothing major or even of the scale they are right now.
The thing with Tobi was, we didn't know anything about him, unlike Isa and Braig. His origin and his motive weren't really known.
Isn't the whole point of Tobi's character to show what could have happened to Naruto, if he went down a different/the wrong path. In this regard Tobi is a dark reflection of Naruto and is suppose to be relatable to both us the readers and Naruto, seeing they're both driven by their emotions.The dark reflection is in my opinion a good archtype for a villain.
In a way even Xehanort is a dark reflection of both Sora and Riku, just unlike Tobi he's not meant to be sympathetic but dislikable and heartless.
When the next saga comes around I wouldn't mind seeing a villain in a similar fashion to Tobi, if it's done write and not forced.
With what he's done and how he shows no remorse about it I don't think any back story could really make you sympathize with him. He treats everyone and everything as pawns in his master plan. Also we already know quite a bit of his back story, most of it's in his reports in BBS.