Fanfiction that follows the course of the events in the game is pretty worthless, eh?

As for Auron not really caring about Rikku in the game- well, he doesn't care about ANYone, really. Tidus and Yuna, perhaps, because he plays a guardian role to both of them.

The event at the Thunder Plains is just one of many in the game... and Auron's feelings could very well have changed. Do you think he WOULD be affectionate toward Rikku if he had romantic feelings for her? No. The way he reacts to her is just a part of the cold front he portrays unbiasedly to EVERYone. He very well could have had romantic feelings for any of the characters in the game, but there was never the opportuniy for him to act upon it. That's what fanfiction is for. ^_^