Not my personal immediate priority, but one of our ECS SMEs (subject matter experts) is currently experimenting with relations. The earliest it would land is probably two releases from now (around 4 months from now), but it could be longer. There is lots of demand for relations (or something like them), and our ECS dev team is very interested in building them. This will almost certainly happen in the "medium term".
(subject matter expert: a formal role in the Bevy project for experts in an area that have a say on big/controversial changes)
Flecs ECS already supports this: https://github.com/SanderMertens/flecs/blob/master/docs/Rela...