I think that the Bladebound Magus archetype could easily be converted into a DM controlled Magus “progression” Arcana.
The only difference between the Bladebound and the regular Magus is that their arcane pool is smaller and they have a magic sword (which is both helpful and not helpful as it's intelligent and has its own agenda).
Functionality:
When, or if, a Magus character can gain a black blade is up to the DM, with or without player knowledge, following the rule that it must be at a level when the Magus gains an Arcana.
The black blade absorbs some of the Magus's arcane pool (as the reason why the Magus's arcane pool shrinks) and awakens at the appropriate level of strength.
“This Arcana changes the Arcane Pool class feature and replaces the Magus Arcana gained at the gained level.”
This could potentially make the Magus character (who can be of any archetype) a bit Over Powered, which is why it's completely under DM control, so if the character is already powerful enough for the campaigns they play they don't receive a black blade. But if the character is under powered the DM can grant them this handicap.
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