The answers may have come through Ant-Man and The Wasp’s mid-credits scene, where Scott Lang was left in the Quantum Realm, which has been teased to have time and/or reality traveling capabilities.
If Ant-Man starts going through wormholes, perhaps it’ll transport him to a time where the Ancient One is still alive, before Thanos caused his own apocalypse.
Obviously the MCU can’t continue without characters like Black Panther and Spider-Man, but it’s unclear if/how the fallen heroes will return.