There's a mod called Invasio Barbarorum, which effectively converts Barbarian Invasion into a full game. It's professional quality, super hard and incredibly detailed, but the best thing is probably that it really makes barbarians unique. Instead of each faction just having a couple of special units, now everyone has unique rosters. Vandals and Franks have killer rosters and they are like the best reason to play. Not to mention Romano-British being playable. And Perso-Armenians. And White Huns, that actually buddhists.