I agree RHR,
a mini set each of Carolingian infantry and cavalry would be nice! Another enemy of the Vikings.
but the other suggestions are also good: Steppe Nomads for the Dark Ages are completely AWOL in 1/72 plastics. Maybe a set of light horse archers and another of heavy lancers? Most of these guys would be fairly universal in appearance. I think hair-style was the most distinguishing feature between nomad groups-various scalp-locks, long hair, etc
I've thought about taking Tumbling Dice Normans and turning them into psuedo-Carolingians by exchanging there kite shields for round shields. They won't be perfect because the distinctive Carolingian helmets from the manuscript illustrations would be missing.