The addition of regional units helped the overall civ design in aoe2. It seems like something that could very easily be added to aoe4 given how the current tech tree is setup.
As is, civs in aoe4 get their uniqueness not by a lopsided tech setup (mostly), but by additions of specific civ perks forming different comps with the unique units. In the future, when more civs are added, there’s inevitably gonna be more and more overlap - so the concept of regional techs/units might be a good way to smooth out the design of new civs.
To illustrate with some examples:
HRE
Regional techs
- War Hammer - common with French for melee anti armor focus
- Two Handed Axes - common with future Danes/PLC for shared Germanic ancestry
- Harquebuses - for all European civs + Ottomans because the took it farther
Regional units
- Black Rider - shared with possible future Hungarians, Bulgarians, and PLC since east Europe had a terrain that needed a medium cavalry to function in the area while also being durable enough for incoming Mongols/Turks.
English
Regional techs
- Spaulders - common with French since English knights would fight on foot more and thus needed more plate cover
- Falchions - common with possible future Celts
- Rangers - common with possible future Celts
Regional units
- Longbowman - common with possible future Celts because Celt tribes were apparently using them in prehistory.
Alternate example
Delhi
Regional techs
- Laminated Bows - common with future South Asian civs (Koreans, Vietnamese) because they were found in the area by Alexander TG and then all over the region by the Europeans.
- Bamboo Construction - common with South Asian civs
- Decimal system - common with Abbasids, because I’m running out of ideas
Regional units
- War elephant/Tower elephant - common with future South Asian civs; as is - Delhi can get a different UU.
Mongols
Regional techs
- Two handed mace - common to Ottoman and Rus
- Cavalry Sabers - common to Ottomans and Rus
- Lamino-lamellar - common to Chinese and Ottomans
Regional units
- Horse archer - Common to Asian civs + eastern European since everyone in mainland Asia had to become the mongols to beat the mongols.
The techs are just to make a point - not things to actually add to the game as they’re written. The main idea; HRE and ENG both have common techs for French since they both interacted with them - but HRE has common techs/units with Danes/PLC since they’re neighbors and ENG is the same way with Cetls.
That’s how regions might be adapted to aoe4 - learn and share more with the closest neighbor, then the next closest and so on. This way we could have civs share similar techs without bloating the number of unique techs overall and also pave a way for future civs to fit in.