Okay, so I was just curious...
Trace can copy any ability. So say it was a Double Battle, and I had out Gardevoir and Lucario (with Steadfast) and my opponent had Ditto (with Imposter) and something else. If Imposter copied Lucario... and then Gardevoir Traced Ditto... would it trace Steadfast or Imposter?
Now, if I had out a Gardevoir and my opponent had a Zoroark disguised as a Weavile with Pressure... would Gardevoir trace Pressure or Illusion?
Trace cannot copy Illusion or Imposter, according to Bulbapedia.
Also, Illusion does not change any aspect about the Pokémon except appearance; in your example of a Zoroark disguised as a Weavile with pressure, Weavile's ability would not be in effect, so Trace would always see the opponent's ability as Illusion, and not activate.
As for Trace post-Transform on an Imposter Pokémon, I just tested it, and, as it turns out, Trace will copy the ability of the Pokémon Ditto transformed into.
I have played a double battle game in Pokemon reborn my side having delphox, Ditto.
Opponents Pokemon is Gardevoir & Meta gross
Ditto transforms into metagross
Gardevoir traces ditto's imposter and transforms into a delphox.