My original X team consisted of:
Pyra the Delphox (F)
Grace the Gardevoir (F)
Mary the Ampharos (F)
Sage the Venusaur (M)
Vlad the Crobat (M)
Scott the Lapras (M)
Anyway, to answer your question, I'd recommend a Grass type to help with Fennekin's Water, Rock, and Ground weaknesses. Venusaur, Roserade, Vileplume, Gogoat, Trevenant/Gourgeist (if you can trade) and Ferroseed are all some good choices. I also recommend a Water type, since they're great in-game and makes a nice Fire-Water-Grass core. Azumarill, Gyarados, Lapras, Vaporeon, Clawitzer, Barbaracle, Tentacruel, and Starmie are some good ones.
Other than those, all you really need is a Flyer, and maybe an HM user if you don't want to waste the bad HMs on your party Pokemon. You could also use a Mega and just wreck everything with it. Also, feel free to try out any Pokemon you feel you might like. Almost anything is great to use in-game. Hope I helped.