I think we should add a list to the OP of optimum Trace switch ins for Garde, with the top ones that benefit her even after Megavolving, and ones that benefit more without.
Heatran: Trace Flash Fire and boosts HP Fire to 90 BP if you choose to run it.
Gyarados, Landorus-T, etc.: Trace Intimidate granting a smoother transition and often prompting a switch allowing for a Sub, WoW, Thunder Wave or Calm Mind.
Klefki, Sableye, etc: Prankster. OMG you can have some good fun with this one, especially since Gardevoir is naturally faster than many pranksters outside Thundurus. This allows you to EV in bulk while throwing out priority Thunder Waves, Substitutes and Wishes and be a real pest. Plus, priority Destiny Bond!
Clefable, Alakazam, Reuniclus: Magic Guard will make all passive damage null. Needless to say this is fantastic for a stall/status set.
Clefable, Quagsire: Unaware lets you safely boost in front of either of these and you should have an easier time taking care of Cosmic Power Clefable, and then you can Megavolve afterwards with your boosts in tact.
Mamoswine, Venusaur: Thick Fat adds some extra resistances for better stalling.
Goodra: Sap Sipper provides great immunity to powders and Leech Seed.
I could go on and better in depth.
Heatran: Trace Flash Fire and boosts HP Fire to 90 BP if you choose to run it.
Gyarados, Landorus-T, etc.: Trace Intimidate granting a smoother transition and often prompting a switch allowing for a Sub, WoW, Thunder Wave or Calm Mind.
Klefki, Sableye, etc: Prankster. OMG you can have some good fun with this one, especially since Gardevoir is naturally faster than many pranksters outside Thundurus. This allows you to EV in bulk while throwing out priority Thunder Waves, Substitutes and Wishes and be a real pest. Plus, priority Destiny Bond!
Clefable, Alakazam, Reuniclus: Magic Guard will make all passive damage null. Needless to say this is fantastic for a stall/status set.
Clefable, Quagsire: Unaware lets you safely boost in front of either of these and you should have an easier time taking care of Cosmic Power Clefable, and then you can Megavolve afterwards with your boosts in tact.
Mamoswine, Venusaur: Thick Fat adds some extra resistances for better stalling.
Goodra: Sap Sipper provides great immunity to powders and Leech Seed.
I could go on and better in depth.