The model of covering overlap in military with regional units was a pretty good one. Extending that, it might be worthwhile to cover technology overlap.
We have UT’s for age 3 and 4 at the castle, so maybe something for age 2 or 1 with the tech located at its relevant building.
No clue what the costs could be tho.
Sample:
Western Europe:
Infantry shields - +10 hp for barracks units(age 2 at barracks)
Central Europe:
Mountaineers - buildings have +3 LoS (age 1 at outpost)
Eastern Europe:
Hill Forts - palisades have +100 hp (age 1 at palisade gate)
Southern Europe:
Gladius - milia line +3 vs spearline (age 2 at barracks)
*North Africa (Berbers):
Rawhide shields - Scout line has +1 melee armor (age 2 at stable)
*West Africa (Malians):
Adobe architecture - build speed of villagers +10% (age 1 at mining camp)
*East Africa (Ethiopians):
Poisoned arrows - archers and skirmishers +1 vs scout line (age 2 at archery range)
West Asian (Middle east):
Waterworks - Farms reseed in 5 seconds (age 1 at mill)
Central Asian:
Migration - Animals do not decay (age 1 at mill)
South Asian:
Crucibles - Barrack and stable upgrade cost reduced by 33% (age 2 at blacksmith)
Southeast Asian:
Butted mail - replaces loom, with an additional +1 melee armor (age 1 town center)
East Asian:
Casting - Barrack and stable train time reduced by 20% (age 2 at blacksmith)
*The in-game region is just africa. Division are only futureproofing.