With only 10 civs, i’d focus this only in Europe and the Near East, and then go for regional DLC’s.
-Brittons (Jack of All Trades)
-Franks (Focused on concentration of force and decisive battles, hits you with everything at once)
-Hispanians (Uncentralised bases, defense in depth, offensives based on raids and harassment, militarised religious orders mechanics)
-Germans (Economic boomers, mercenary mechanics, will snowball if unchecked, maybe a mechanic where they established allied AI controled bases to mimic the decentralised nature of the HRE)
-Italians (Solid turtle economy, can afford to run on a single base, to mimic the italian city states) and loose map control and still keep an economy running, mercenary mechanics)
-Slavs (Dominates map control, some mechanic to make expansion easy)
-Byzantines (Noob-friendy, Great early game turtling and economy, almost imune to raiding)
-Sarracens (Good map control, great early game economy, mechanics to baiting you into a war of attrition like trash wars.)
-Mongols (Entirely mobile base)
-Turks (Unmatched late game siege and offensive capabilities)
Possible campaigns would be:
100 Years war (Franks, Brittons)
Reconquista (Hispanians, Sarracens, Franks)
Italian Wars (Germans, Italians, Franks, Hispanians)
Mongol Invasions (Mongols, Slavs, Sarracens, Turks, Germans)
Crusades (Franks, Brittons, Sarracens, Byzantines, Turks)
It would be cool that if they used a Age of Mythology minor-god approach when aging up.
Each civilization could have sub-cultures within itself that gave you different bonuses and different units.
Like, if the Brittons could choose English Longbowmen, Irish Kerns or Scottish Galloglaich.
The Hispanians can choose Castillian Knights of Calatrava, Aragonese Almugavares or Portuguese Conto Crossbowmen.
The Slavs could choose between Polish Hussars, Muscovian Boyars or Cossack Cavalry.
Etc…