So I have spent some time on yugioh master duel and they have a feature that I really appreciate: they have some single player missions with pre-defined decks. Usually a short guide about core combos of a particular deck build, then a mission using that deck so you get familiar with its playstyles.
Of course yugioh has a million more decks (and the current sp content for master duel is far from enough) and AOE3 is very different as a game. But some ideas could be borrowed.
One of the big reasons I usually see why people cannot get into AOE3 is the overwhelming variety of options. Card selection combined with timing seems too difficult for one to figure out by oneself. Not to mention the civs are vastly different. Art of war missions are great but they still are limited to basics of the game.
I as a long-time player even got confused for a while with newly-added US/Mexico/Africans etc. So having some civ-specific “training” missions with pre-defined decks and some guidance would be beneficial for both sp and mp of the game. And they can also be easily incorporated into new campaigns and historical battles.