As a kid, I had the WRG Ancient Army Lists (with the "Confused Roman" on the front and a selection of 5th edition armies within. Some of the armies, "Barbarian Conspiracy" and "Seleucids", were a mystery to me but I spent ages poring over this slim tome and totally failed to apply a realistic approach to recruiting armies.
Those were the days when model soldiers for me meant mostly converting Airfix plastics. With no clear idea of how I would ever approach the hundreds of figures needed for a reasonably sized army, I converted US Marines into Sassanid spearmen and American Indians into Cretan psiloi. I based up a whole box of Ancient Britons as "Irr C LMI" but then found that they included bare-chested axe-men with top knots. A perusal of the lists (and finding a quotation from Tacitus in a book in the school library) convinced me that these few figures had to be Early Germans (another WRG label). And so they were based as "Irr C MI" on smaller bases.
All of these diversions meant that I never got close to finishing a 5th edition army. We played a few games but only with friends' armies and bits and bobs of mine. Soon after, I got hold of Bruce Quarrie's green Airfix guide rules. That started an attachment to WW2 gaming and pretty much ended my flirtation with ancients.
Hordes of the Things doesn't have armies lists of the type you aspire to. An army is small and relatively easily completed. Having struggled for some time to decide how to organise my Border Reviers, I recently had a brainwave. Should they be Riders - fast moving but no good in bad going? Or should they be Beasts - slightly slower, easily defeated by other mounted troops but more effective in Bad Going? Beasts made a certain sense but I had "Slewe Dogs" in one of the forces and they had to be be Beasts. If the mounted Reivers were Beasts too they would lose their special "feel".
The Brainwave was this. The Reivers would be Beasts but the pursuing Warden's men would be Riders. All though the two forces are made up of similar troops - the same people riding in pursuit of Reivers this week may be leading their own raid next - the Reivers are slowed down with booty! If caught driving cattle and sheep before them they will be less able to resist their pursuers!
Well, it works for me anyway. Now back to concentrating on not starting any new armies!