I'd like to see Ash catch a member of the Ralts line. I know that Max befriended a Ralts and that the Ralts line is very attractive, making it great for contests, but I think that it would also suit Ash very well.
As for how he would catch it, there are a number of possibilities floating in my mind. One would be for a battle bred Ralts to be with a family who is not focused on battling, but on show business. They'd be pop divas, all save one, Ralts's owner. Ralts' owner, however, isn't enamored with singing or the stage and she would be be secretly training Ralts in egg moves, especially Destiny Bond. She'd encounter Ash and battle him and lose, but put up a good fight. To make a very long story short, Ralts and her owner plan for TR to capture Ralts. They do so, once TR gets out of there, after having the rescue attempt thwarted by Ralts herself, Ralts turns on TR and blasts them off. Eventually Ash finds Ralts, but doesn't know its the same Ralts that was captured. He captures it and shows it to his friends. Ralts's owner recognizes her Ralts, but Ralts doesn't want to go back at all. Ash makes ready to hand Ralts back when Ralts's owner refuses, saying that Ralts has made her choice. Ash then welcomes his first psychic pokemon to his team.