One well-backed Final Fantasy 7 Remake theory suggests Aerith isn't the last of the Ancients, as previously mentioned multiple times throughout both Remake and the original Final Fantasy 7. This article will focus on one NPC, in particular, who fits the Cetra description.
[Warning: SPOILERS for Final Fantasy 7 Remake, Final Fantasy 7 & Advent Children below.]
The Ancients (Cetra) are believed to be an ancient civilization with the special ability to interact with the Lifestream, and in FF7 Remake, they're described as the planet's stewards. In the original game, Sephiroth calls Cloud a traitor at the Shinra Mansion, explaining that the Cetra were a migratory race that would settle in different parts of the planet to heal it before moving on to another location. Yet some thought the journey too hard and decided to settle permanently. "They took that which the Cetra ... had made without giving back," according to Sephiroth, making them and their descendants traitors.
Sephiroth's description implies humans are descendants of Ancients who chose to cease itineration and, as a result, were no longer protecting the planet. If those who forwent the Cetra journey became humans, could the role be reversed? Could a human become a Cetra upon learning the Ancient ways? YouTuber Final Fantasy Peasant seems to think so and has provided much evidence as to how this could be happening to one seemingly minor character: Barrett's daughter, Marlene Wallace.
Final Fantasy 7 Theory: Marlene Is A Cetra
The relationship and similarities between Marlene and Aerith are a big clue, especially in other Compilation entries, like Final Fantasy VII: Advent Children, which takes place two years after the original game. In Advent Children, Marlene wears her hair exactly like Aerith's. Her role in the film strongly mimics Aerith's, too: She's sweet yet straightforward, she provides the opening narration (similar to Aerith's face-zoom in the intro to the games), and she takes over Aerith's tending of the church flowers, which is seen as a representation of the planet to many. Surely, this dense planetary exposure has some kind of effect on the girl. And at the end of the original game, when the camera pans to Marlene sitting alone at a table before the Lifestream erupts from the ground, she knowingly says, "The flower girl?," despite meeting Aerith just long enough to be taken to Elmyra's house when the Sector 7 plate drops.
Final Fantasy Peasant's theory came out before Remake, yet the 2020 game only enhances it. In Remake's haunted trainyard scene, Tifa sees a crying Marlene, while Aerith sees her younger self crying. After Shinra takes Aerith in Remake, Marlene tells Barrett, "That girl... She was kinda...," and drifts off in thought. Barrett asks, "What?," and she immediately says, "Nothing. You should help her." Both girls know more than they divulge. This mysterious connection with Aerith segues into other times when Marlene knows more than she should, like Remake's ending, where she's tending to more flowers and, for an unknown reason, senses her father. She has a similar scene in Advent Children, referring to Aerith again.
Perhaps the hug between the girls in Remake transferred some kind of Ancient knowledge to Marlene. Whatever happened, she became calm and trusting of the flower girl. Final Fantasy Peasant mentions that one of the Final Fantasy Ultimania books (canon guides and extras for the series) states that, when Aerith takes Marlene to Elmyra's in the original game, she lets her briefly hold the Holy Materia - something no one else gets to do - explaining that it used to make Aerith feel safe when she was a child. This could be another way the Cetras connect to Marlene.
In Advent Children, Marlene also knows Cloud's Geostigma disease is located on his arm after Aerith appears to him and touches it in the Ancient City. These non-physical connections between Marlene and Aerith hint at Marlene taking her place as a protector of the planet. Final Fantasy Peasant offers a lot more evidence in the video, but it seems Square Enix is tying Final Fantasy 7 Remake with other FF7 Compilation entries, and it'll be interesting to see how this plays out in future installments of the game.
Source: YouTube/Final Fantasy Peasant