Maybe an update of the Airfix Tarzan set, but with more natives carrying baggage etc. who could be used as bearers for colonial armies, rather than animals.
One problem with Dark Ages sets is that it's difficult to establish identifiable appearance for them; what did Brian Boru's army look like? My guess is that they'd look little different from their Viking enemies (and allies!). In fact virtually any European army of the time may have looked much the same.
That is a very valid point reguarding the dark ages as well as other periods. My answer would be to pick one distict style of the time, arbitrarilly assign to it a nationality and cantury, then produce the a set as if it were the "uniform". Allowing of course, some variations for rank and things like that.
Sort of like doing a set of Romans exclusively in scale armour and calling then calling it the eastern legion, another in plate armour and calling them preatorians, still another in scale armour, segmented armour and so on.
As long as enough sets are done to cover the major/significant styles.