OK, first of all, I need to see your moveset if I can evaluate your team. I can't make much suggestions beyond this till you add in movesets.
Now, for the extra pokemon.
Ok, your team has extraordinary problems with ice, so I'm going to suggest not using Garchomp. Gardevoir would be good if you managed to breed Will-O-Wisp on to it to deal with T-Tar and Weaville. Don't bother using Dialga - he's Uber, while your other pokemon are OU or UU. Alakazam would work as a lead, but unless you're using Double Screen Encore/Trick, it's going to be outshadowed by Azelf thanks to Explosion. Gallade is a good option, but a tad slow, so you would have to have bread Shadow Sneak on him, as well as have Swords Dance (no problem, it's a TM). However, one pokemon you may have overlooked is Scizor. To be truthful, I dislike Scizor very much, thanks to the fact that he is way too powerful for OU currently. However, that doesn't matter - he works well with Swords Dance and Bullet Punch.
You should probably switch off Luxray - there are pokemon that can do the job he's trying to do better. If you took Scizor, use Zapdos in replacement of Luxray.