Array foreign keys and indexes on arrays/JSON nested fields. When using MongoDB, having this eliminates most need for join tables. (*This may have already been implemented, I haven't used SQL recently.)
I think they have indexes on json (or any selectable thing?).
You could probably implement the foreign key using generated columns these days. Ah I guess you mean instead of having the association table? That would definitely be nice in some cases.
We very occasionally use integer[] columns with ids, but hold off on using them widely b/c of the lack of integrity constraints. It'd be awesome to use more often.
