Do you think the campaigns might need significant changes? Or slight changes from civs being buffed?
There’s balancing around multiplayer but then there’s also the campaigns. Luckily it’s much more easy to balance a campaign since you can use triggers or place down more relics/resources to give the human player more help.
The Horde Rides West for example might need some changes. The Mongols now face a much stronger Persian AI. Throwing down a relic (or more resources) near your main base might be enough. Or some helpful trigger to help the Mongols beat a now much stronger Persians.
To be fair, as a campaign creator, 3 difficulty levels, done in a comprehensive manner, are already a lot of work to parameter. Campaigns used to have 5 difficulty settings in the past, most of them not relevant, as setting all of these was already a pain in the ass, and nowadays it would just turn AI scripts in novels.
I agree that an Exterme/Hardcore level past hard could be worth it for super experienced players, but most of the SP playerbase already struggles on Hard difficulty tbf.