I am not too sure how possible or practical this is, but it would be nice to gate certain drops behind advancements. For example, I already received 2 elytras in my world from captain drops, but I havent even been in the end yet.
So for example make diamond tools drop only after the user has the diamond advancement, elytras only when the user has the end advancement.

That's a great Idea that I didn't think of, I will be sure to consider this - thanks!

Rather than posting another suggestion, I suppose it would make sense here: I was thinking a config option to have them only spawn after the ender dragon has been defeated for the first time (this could be done by triggering to only start once a player gets free the end for example, or could use SeenCredits nbt)