This should be introduced. Even some mods considered official. Certainly, modders would like to contribute to the game (by the way, small financial earnings)
Co-op should be introduced to the campaign even as a paid add-on. People would certainly support such an idea.
Time should be stopped with a limit of a minute or two where there is an automatic restart. Time should fly all the time in slow motion (acceleration only with the consent of the other player). The difficulty level is slightly higher, of course, because a possible alliance of 2 players would be stronger.
When two players are in cities or inventory time automatically pauses indefinitely.
(and this editor so that you can create an additional culture or possibly a wife or brother)