Subject says it all. Is there any path wherein a improved familiar can be a cohort (and advance as one)?
So, basically Monstrous Companion for familiars?
...I think the closest thing to that are some teamwork feats that give you some of the familiar benefits (Bonded Mind - > Share Spells - > Special Delivery) with an ally. You and your cohort could get those (the cohort only needs Bonded Mind) to get Share Spells and Deliver Touch Spells.