The entire dialogue between Aerith and Cloud in Cloud's subconsciousness firmly implies that she intended to put an end to Sephiroth's ambitions in place of Cloud and come back to him. She realized what was needed to counter the Black Materia and since Sephiroth had Cloud give it to him, she figured the best possible way to stop him was Holy.
Cloud
What is this place?
Aerith
This forest leads to the City of the Ancients... and is called the Sleeping Forest.
It's only a matter of time before Sephiroth uses Meteor.
That's why I'm going to protect it. Only a survivor of the Cetra, like me, can do it.
The secret is just up here.
At least it should be. ...I feel it. It feels like I'm being led by something.
Then, I'll be going now. I'll come back when it's all over.
However she didn't anticipate Sephiroth eavesdropping on their conversation and getting a heads up on her plan to stop him. Aerith didn't see or sense Sephiroth's approach at all until she was killed.
However, Aerith's death does turn into a makeshift blessing in disguise, because it is due to her ability as a Cetra and not diffusing into the Lifestream, that she's able to rally Lifestream to hold back Meteor once Holy on its own was insufficient in stopping it.
The ending of FFVII is pretty obvert in revealing Aerith's hand in thwarting Meteor's impact on the Planet. When Meteor begins bearing down on Midgar after Holy fails to shield the Planet from its approach, the Lifestream rises out of the ground out of nowhere and begins to hold back Meteor and works in tandem with Holy to destroy Meteor and preserve the Planet.
Aerith face then appears and smiles. Unless you believe Marlene's sensing of "the flower girl" and her appearance upon the interjection of Lifestream to stop Holy was just elaborate coincidence, there's a very strong implied meaning there.
Furthermore in subsequent guidebooks for VII, it specifically references Aerith rallying the Lifestream to stop Meteor and aid Holy which was not strong enough to stop Meteor on its own. Aerith had a very specific part she played in saving the planet.
