So, if we're talking about a good 6th Pokemon rep... Well, I think Pikachu, Jigglypuff, Pokemon Trainer (Kanto, mind you), Lucario, and Mewtwo would need to be in there first. I think they take top priority over any other possible addition.
So as to what that theoretical 6th rep could be, that's rather tricky. I might say Meowth, though his presence outside of the anime is rather weak at this point. I think he would have made perfect sense as the second Pokemon rep in the original game, but at this point... I'm just going to say that the fact that it's literally impossible to catch a Meowth in X and Y without the use of PokeBank says something. Zoroark makes a lot of sense as a 5th gen representative, and he'd certainly be unique, but I don't think he quite matches up to Lucario in terms of being an icon of the series. Another Trainer could work, but I don't think any of them match up to the original one.
...Picking a 6th Pokemon rep is hard...