Let’s try to be historical : the late roman empire still had infantry at its core, supported with cavalry and scorpions, but also relied on germanic mercenaries. When you’re facing several germanic group, sometimes the best is to throw gold at one of them to fight the others.
It also was an empire in decline so I don’t think it should get top-tier units.
Romans : infantry and defensive civilisation.
UU :
- legionary (castle) : rework it from its current version, giving it a ranged charge attack, throwing a plumbata before attacking in melee. This charge attack is armour-piercing
- field scorpion (siege workshop) : replaces the scorpion, looks like a roman scorpion instead of a ballista. Cheaper, fires armour-piercing bolts, but a more limited overperforation than the scorpion (enough to impale 2-3 infantry in tight formation). Intended as a replacement for both the crossbow line and the scorpion, it provides general firepower behind the meat shield that is your infantry.
UT :
- mercenary auxilliae : unlocks the auxillia barracks building
- diocletianic reforms : infantry (barracks & legionary), and cavalry (stables) cost -15%, trained 25% faster.
UB :
- auxillia barracks : allow the recruitment of other players’ unique units, plus their elite upgrade.
(For example you can recruit huscarls if another player plays Goths, throwing axemen for Franks…)
Bonuses :
- buildings get +1/1 armour per age, up to +4/4 in imperial age.
- infantry counts for -10% population in castle age, -20% in imperial age.
- knight line +10% speed
- lumberjacks provide a little bit of stone
Team bonus :
- knight line +5 damage vs siege
Significant missing techs and units :
- crossbowman & arbalester
- hand cannoneer
- paladin
- camel line
- scorpion & heavy scorpion (replaced by field scorpions)
- bombard cannon
- bombard tower
- siege engineers
- infantry 3rd armour
- cavalry 3rd armour
The stone and building armour bonuses will allow solid defences, so long that the enemy does not bring siege weapons, and building fortifications everywhere such as a wall the entire width of Scotland. You’ll need that, as your empire is in decline, and being before the age of plate armour, gunpowder and even crossbows that can do more than hunting small game, you will lack most shiny imperial age units.
This means the core of your army will be infantry, including your elite legionaries, supported by field scorpions and lighter, but faster, knights (mainly to counter enemy sieges and archers as they lack the brute force to beat heavy cavalry). Trash units remain decent when gold is scarce, though nowhere near top-tier. The boldest generals might be tempted by the option of hiring mercenaries to fight fire with fire.