It is possible that Paean and Paeonia and its inhabitants, the Paeonians, are connected. The Paeonians were a Northern tribe present in the Iliad, sided with the Trojans. They lived in the place that later became Macedon. The hypothesis of some form of connection with them and Paean or Apollo in general has been suggested, rejected, re-suggested and re-rejected only to be suggested once again.
It may be possible that Paean is an adjective, meaning "of healing", since it has been associated with Apollo and Asklepios, both healing gods, and when used in the Iliad alone also refers to a healing god. Some scholars say that maybe then this was the title given to the healing deity of the Paeonians, which seemed to be their patron male deity, possible Helios, Apollo or some other God associated with the sun, who was worshipped along with Artemis, associated with the moon.
Not much is known about the Paeonians. We know their religion was centered not only on the sun and the moon (as Apollo and Artemis), but also on the rivers and lakes. In fact, the Axius, a river, was the great-grandfather of the chief Paeonians in the Iliad. From the Iliad we also know that they used to wash their to-be-kings in the water of this river and that would make them kings.
Herodotos says that they offered to Artemis offerings in the same fashion as those sent from the North to Delos from the Hyperboreans. Another clash with Apollo's cult is the fact that they worshiped Helios with rituals similar to those performed in the Daphnephoria.
Their chief god, whomever he was, was linked with not only the sun but also with water. He was a great healer, the god of healing waters and, mostly, of healing herbs, just like Paean and, not surprisingly, Apollo.
It appears that the Paeonians themselves were known for their salves and their knowledge of healing herbs and that Paeonian could also mean "doctor", as iatros.
Another curious thing about the Paeonians, one which I find beautiful and easy to incorporate in modern devotion and cult of Paean, Apollo, Helios or Asklepios, is that they were refered as the famous Northmen of famour pharmakoi and their land was in the road of rose gardens (because in their land grew roses and peony).
One last thing, besides Paean, Apollo and Asklepios, another healer linked with them is Chiron, whom they taught how to heal to Asklepios himself.