Well I know in my homebrew setting Barbarians are Vodoun Mambu (Priestess) and Hungan (Priests) who have let enranged and wild Loa enter their bodies and "ride them".

So I think Vodoun-worshipping, 18th century-esque appearance/mixed with Louisiana Bayou is pretty different from Viking and Celtic.

I find if you emphasize the whole spirit-connection more then the "I SMASH" aspect your can extremely broaden out the concept. Since well spirits exist in basically every culture.

I think I should be able too, the Rages can become the Trances that Vodoun-Priest/ess go through which can be very erratic and sometimes to the untrained eye quite violent looking. The Rage being used at the time can symbolize a different Loa entering the body and taking over at that time.

The normal Powers, are when the Loa by overpowering the bodily control of the Priest it can push him beyond normal means, ie; you can have what looks like a weak Priest perform a devastating cleave. Same goes with Barbarian abilities where you shrug off hits or gain temporary hit points, since the Loa is commanding the body it doesn't sense the pain.

With more supernatural Powers that is when aspects of the Loa actually leave the body and perform deeds around it/through it.

Roleplaying wise a Player can pick Powers that suit a certain House of Loa, and have it so his priest devotes his efforts toward that house; thus he has the gift of being ridden. I imagine a Barbarian in this setting would probably be covered in talismans and concoctions to allow him to quickly enter a trance so he can be taken over, and perhaps smelling salts to awaken him afterwards.