Could the belgae be given more land in Britain? Or even Ireland? I'll explain as I know it may not be historical.... The Late Game
And maybe this could be applied to giving some better lands further to the edges of the map for other factions also and giving those regions big bonuses to income,, BIG BIG bonuses.
No matter what faction I play, and this frustrates me most as the Romans as the end game is just waay too easy to truly enjoy all those fantastic legions I have deployed protecting my border forts. All so prettily arranged whose legates hold ancillaries like surgeon, battlefield messenger, master of the archers of crete and Legate on LEGIO whoever, to guard against................dead factions, rebels, factions too scared and most definately incapable of fighting back. Having these uber settlements at the edge of the map could allow the belge in the north west, sarmations north east, gallaeci/carthage south west and selecuid/ (preferably parthians) in the east to keep kicking back.
I know there's some problems attached like being unhistorical, not realistic, might bolster a faction which becomes huge anyway and now have an unfair advantage and huge secret treasury at their back which is only supposed to be used as a get out card. However I think it might keep a heart beating and motor running right throughout till the end game, as long as it doesn't interfere big time with the early game.
As well as this whenever I conquer southern britain the belgae die off as they've no family members in ireland. So just for the sake of keeping factions alive longer.
Maybe these bonuses would only be under conditions and specific to faction. eg bonuses in caledonia Hibernia specific to factions like Belgae, Averni, Cimbri, Boii if <5 settlements.....bonuses in east settlement specif to parthia/ seleucid < 5 settlements and so on. Don't know if this can be done.