For example:
Before birth. Add a notice board or NPC near the NPC who names newborns, displaying a newcomer seeking adoption. This puts them in a waiting state for other players to adopt, until someone adopts them or they give up.
Also, add a notice board for players to show a list of characters not yet born who are looking for others to play with.
Or, add a random adoption feature to the Wilderness Beacon.

Before the player leaves, choose an heir. Inherit everything.
This will definitely double the number of online players. Triple!!!