Frost2779
commented
6 years ago I think one problem with that fix would be that spells that qualify would have to be implemented twice. Another solution would be for it to add spells that qualify to the list like with normal 5th level or lower spells.
The Warlock ability Mystic Arcanum currently does not allow you to modify it in any way which means new homebrew spells that qualify cannot be selected as a Mystic Arcanum. This could be fixed by implementing a Mystic Arcanum builder similar to the Eldritch Invocation builder or by allowing the SRD content on the website (such as the base classes) to be edited.