I would really like for Barbarians to have a variety of units and playstyles as opposed to what we currently have, after all there are many different types of barbarians that would be amazing to play as, and they should all come with aesthetic differences. I'm going to give the rough idea, but the unit changes I leave up to the imagination.
Viking Subculture - Passive changed to Battle Vigor, gain health on crits (Or perhaps even increases Morale?) instead of inflicting extra damage. Units are a little tankier with gambesons and chainmail, leading to a more heavily armored barbarian culture, while still maintaining the overall feel.
Hun or Mongol Subculture - A cavalry focused Subculture that specializes in mobile ranged units and skirmishers. I would love to play as a horse-raider type barbarian culture but it doesnt quite feel right with the Barbarian culture's units as is. Part of their passive gives optional cavalry units their cavalry (I.E. Tome units now will automatically get mounts even without mount masters and what not)
Celtic Subculture - Less armor, more mobility, and perhaps even intimidation to their enemies? Painted warriors march into battle wearing very little. Skirmishers and Shock based infantry focus. Perhaps their passives could include giving slippery to all their units? However that might be too strong...
There are many different barbarians in the world with really cool aesthetics and units tied to their cultures, it would be a shame to not get to indulge in that variety with our Barbarian Culture!