In an interview with IGN at E3 2019, Aya Kyogoku and Hisashi Nogami of the Animal Crossing: New Horizons development team confirmed that Isabelle will be in the upcoming game and her appearance depends on your progression.
“If Tom Nook is a great recruiter, and if he succeeds with the Nook Inc. Deserted Island Getaway Package PR, then she may come back,” Kyogoku said.
In Animal Crossing: New Horizons, players start their adventure on a deserted island with a tent and only two other animals rather than an established town. To progress, players must work toward improving the island.
Isabelle’s appearance “may” also depend on how much debt the players have paid back, Nogami said. While players aren’t required to upgrade their tent, Kyogoku said that Tom Nook may not see certain facilities as a necessary upgrade.
Source: IGN.com Isabelle Is In Animal Crossing: New Horizons but There's a Catch