Back when they released this AOE3 artbook I was disappointed, but I saw it as testing the waters.
The majority of users want AOE2 gameplay. The main criticisms of AOE3 were the new home-town features and unsatisfying gameplay stemming from the change to ranged combat over melee. Swords just don't make sense once muskets are a thing, and the counter to muskets doesn't come until tanks at which point you're making C&C. AOE2 defined the series because of the mix of cavalry, archers and infantry leading to a great rock-paper-scissors element you can build a game off. And a game with that formula also including musket warfare and onwards is stretching to the point the game loses simplicity and conciseness : it's not Empire Earth.
AOE4 should be made with the mindset that AOE2 is the series, not just 'the series should be expanded upon'. It's got a good formula with strategic choices, random maps and large civ diversity, a grounding in historic events and technology, a mix of good combat with fast moving units and macro base building. The middle ages is AOE's historical niche which also gives it the series defining gameplay.
As for what could be added, look to starcraft 2 and league of legends. Starcraft 2 for advances in the RTS engine, and league for advances in ability design.
The number of civs should be kept high. The diversity in civs should be greater and I think you could keep general familiarity by segmenting the civs by region more (like with meso civs). So place more emphasis on the uniqueness of say 'East Asia' civs by changing one or two of their base units or buildings for all of them. Then further differentiate them with unique things (which has so much more room to grow than just 2 unique techs and a few bonuses. they have the right idea with the zany shit the last expansion gave, they're just greatly limited by the engine). Once the greater regions are divided into real brackets (not just building sprites) learning all the civs becomes more manageable and you can experiment more with the civs themselves. Think C&C generals zero hour.
Do I want another age of myth? not really... potentially?