Destiny: Frontiers is a codename for an upcoming content release in Destiny 2, scheduled for 2025. It is speculated to be an expansion or possibly the beginning of a new narrative arc. Details are scarce, but it is confirmed to be part of Destiny 2, ruling out the possibility of it being a Destiny 3 sequel.
It is uncertain whether Destiny 2 will receive more true expansions. Bungie has not confirmed any new expansions beyond the recent release of The Final Shape. There are theories that Destiny 2 may switch to an Episode-only model or could be working on Destiny 3. However, expansions have been a significant revenue driver for Bungie, making an expansionless future seem unlikely. More information is needed to determine the future of Destiny 2 expansions.
Catarina Macedo is leaving Bungie after two years as the Project Lead on Destiny 2 expansions. She has not explicitly stated her reasons for leaving, but has mentioned that she will be taking some time off before moving on to build something new. It is unclear whether she will stay within Sony's umbrella or work on an entirely new project.