Anna (Malese Jow) and Vicki (Kayla Ewell) were both killed in the first season, but Jeremy (Steven R. McQueen) was able to see them after being brought back from the dead in the second season finale.
Plec has now teased that Anna and Vicki's appearance in the instalment was "just the beginning".
"We'll get a good sense in the first episode or two how they're manifesting themselves and where they're coming from," she told TV Guide. "As we get deeper into the season, we'll start to understand that they have a compass and they have their own master plan.
"You'll have to wait and see whether or not that's a diabolical master plan, [but] I think the minute you get any supernatural entity dropping into Mystic Falls, somebody is going to be at risk."
Plec also confirmed that the return of Anna and Vicki will cause problems in Bonnie's new romance with Jeremy.
"[Bonnie] went against the order of nature [to save Jeremy] and now there are obviously supernatural consequences for what she did, but clearly there are going to be emotional consequences," she said. "These two girls are girls Jeremy loved very profoundly. Bonnie is a nice, new, warm, wonderful, healthy relationship and inevitably it's always the healthy relationship that gets tested when the [more volatile] relationship comes back."
Plec has previously revealed that Jeremy will still not know what is happening when the show returns on September 15 on The CW.
> 'Vampire Diaries' casts Tyler's love interest
Watch the clip of Jeremy seeing Anna and Vicki again below: