Custom templates is probably the least bad idea of this proposal. Javascript has had this feature ever since ES6 template strings were introduced and I've yet

Custom templates is probably the least bad idea of this proposal. Javascript has had this feature ever since ES6 template strings were introduced and I've yet to see it abused (although I'm sure some folks have). I regularly wish Python's f-strings had custom processors, as it allows creating more convenient safe APIs, instead of having to hunt down idiots who build queries with f-strings because that's more convenient than using the safe-SQL-composition (to say nothing of query builders) they're provided with.