Almost every game whatever the type that has a historical or real world basis (this includes legends and mythologies that are existant in the real world): it starts with the most famous factions/characters within the setting, with straightforward traits and constrained contents. They may be balanced among themselves at the beginning.
And then as time passes, factions/characters that are less prominent got added. To make them unique, the developers have to keep stuffing new gimmicks and more contents into them. Eventually less prominent factions/characters have seemingly fancier traits and much more contents.
In the meantime, the old contents cannot be re-sold again so their update is much slower, on par with regular maintanence.
That’s basically what I feel most long-lasting game series worked. There is not necessarily a power creep, but the result is always the most prominent factions/characters that people immediately think of when talking about the setting, are those with least love and flavor.
A few games that seem to find a way out of this are later historical total war games or paradox games, where they sell unit/content packs for many existing factions. But of course that is not a model that a multiplayer RTS could follow.
Or maybe the pre-steam CD era expansion model, where there is large room for updating old stuff because the expansion is effectively a new game.