So, the thing we're used to seeing with regards to pronouns and case systems is pronouns having more cases than regular nouns: trivially, English has I vs. me, he vs him, she vs. her, etc. Finnish has a completely distinct accusative for most personal pronouns. However, we find exceptions: in Georgian, some personal pronouns merge the nominative, ergative and dative (which also is the accusative).
I was wondering what interesting things we could do with this.
An obvious solution is having the defective pronouns always have the same syntactical role, and voices are used to modulate its semantic role. This could get especially interesting if you also have voices that conserve the desired information structure - i.e. they are not used to make an argument less or more salient, they're just forced by the presence of a personal pronoun.
This could lead to interestingly distinct uses of voices depending on whether there's a pronoun present or not. For instance, emphasizing an argument might be part of the function when only normal nouns are present.
Notice how I said that the defective pronouns always have the same role. This is actually ambiguous: either it means 'there is one particular role A, and each defective pronoun always has that particular role'. The other meaning would be 'there is a set of particular roles A, and for each defective pronoun there is exactly one member of that set that it always will have'.
With that we can probably start getting into some pretty interesting wild notions, where we end up with a multitude of voices that can be combined to switch pronouns of different types simultaneously around the semantic role-space.
2) Permit case / role marking on some different entity.
This is basically sort of the idea that evolved into this post. This could be made very boring: extract the case marking onto a particle that goes somewhere else, so e.g.
I you see particle.me : you see me
In this case, the particle sort of becomes a stand-in for the personal pronoun, in effect doubling the existing personal pronoun. What I want is something more befuddling. So, let's go with this: place the case on a reflexive pronoun!
I see bear > I see the bear
I see bear self.acc > the bear sees me
I see you self.acc > I see you? you see me?
Normally, reflexives only happen with transitive verbs with no other explicit object, thus making their use in transitive clauses a reasonable approach. The only exception is when there's two pronouns present. Of course, some other rule may disambiguate there: proximity, some ranking thing (e.g. the reflexive pronoun always tells us the case of the pronoun highest in this order: 1 > 2 > 3). This may force there to be a nominative reflexive form, which obviously may well be useless in all other positions due to reflexives often not appearing as anything even remotely subject-like.
Further, one could bring in some coordination! Coordination often has some kind of restriction like 'one can only coordinate things of the same type', so e.g. 'I and the girl gave flowers' is not a valid reordering of 'I gave the girl flowers'. Then one could have some semantically empty element that can carry explicit case, and coordinate it with the pronoun that needs the marking. This also permits for separate case markings if needed for multiple pronouns. However, this leads to things like
I and self.nom give you and self.acc he and self.dat for a slave.
3) Have Differential Case Marking on Other NPs
So, let's have non-subject personal pronouns force some slightly odd things going on with other NPs, like maybe making subjects go instrumental or something. So essentially begetting a split-ergative kind of thing, but with the pronouns just not marking for anything.