My new vote:
1. Psychic
Psychics are either special or physical, never in between. The only Psychic that could go mixed with out sacrificing too much is Jirachi, but s/he is mostly used for support or CM sweep. So a mixed Psychic would be great.
2. Fighting
The majority of the fighters are physical, with the exception of Infernape and Lucario, while Lucario goes with the SD set most of the time, and Infernape is mostly Mixed. The only special Fighting moves are Focus Blast and HP Fighting, and Focus Blast's accuracy sucks while HP Fighting is too weak without boosts. So I propose a Special fighter.