A Challenge: Origin of Person Congruence

Can anyone come up with other origins of person congruence than pronouns that are merged with the verb (or, for that part, merging the verbs with verbs that previously have been merged with pronouns)?

I have two ideas, out of which one is not very good.

Reinterpretation of direct-inverse morpheme

Easily, the direct morpheme could be reinterpreted as solely being used when the subject is at the top of the animacy hierarchy, and thus either becomes a first person or first-and-second person congruence marker. (Some langs iirc rank second person higher, so that's also a possibility.)

This even leaves open the possibility of using the inverse marker solely as a marker for third persons, and then an unmarked verb could be second person. Other paths to such a situation can be constructed.

Unlikely rebracketing of case morphemes

Some languages permit omitting the accusative marker on nouns when the subject is, say, a pronoun. (This might assume case marking on the pronouns still obtains or some type of congruence already in place - otoh, Chinese is somewhat pro-drop so why couldn't this work without a pre-existing congruence?)

Now, we can restrict this to, say, omitting case marking in the presence of a first (or second) person subject. See where I am going with this? Now, let's have the case marker - either a suffix or a prefix of the noun - condition a sound change at the word boundary of the verb or just be rebracketed as a subject marker, and then generalized to all persons.

SVO: an object with an object prefix triggers a change, causing a verbal suffix
SOV: an object with an object suffix triggers a change, causing a verbal prefix

Of course, a thing that could further influence this could be a split-ergative system, where absolutives and ergatives and accusatives cause different things to be rebracketed with different persons as subject.