I sort of asked the same question earlier this year. I wanted something that could handle from 400AD to 600Ad, scaleable enough to handle hundreds of minis, good unit differentiation across factions, and ideally would have characters for a campaign.
I've come to conclusion the WAB Age of Arthur is still probably the best ruleset to start from. In our house rules, we've added in some components from other WAB supplements such as Fall of the West so that it's not too Britain focussed. I've also bought Dux Britanniarum, Dux Bellorum and Comitatus. They are all good but WAB still seems more flavoursome! We're currently in the early stages so typically only have a few hundred minis per side but it seem to work fine with units typically of 24 minis for infantry and 12 for cavalry.
The Midgard rules from James Morris seem like they also might fit the bill but aren't out yet as far as I know.