The question if it’d be possible to expand the modifiers via addons comes up occasionally because it’d be a nice thing to have but Python runs simply way too slow to handle the amount of vertices in most cases.
I wrote a smaller addon myself which already shows signs of delay with just 100k vertices in the scene, so yeah, via addon is out of the question.
But what about loading them as DLLs? These should theoretically run as (or atleast almost) fast as the internal code, so would it be theoretically possible to supply new modifiers that way so we could finally get the barrier of custom builds out of the way atleast for modifiers?