If Age of Empires 4 is around the time of WW1 and WW2 I have a few ideas that might make the game unique in comparison to games that already exist. It might not be much age of empires like, but I think it will be a good addition to the game.
In my opinion it would be nice if you have gamemode with a world map or the map of Europe with many smaller zones. A coastline zone, country side zone with villages and small cities and a unique Mayor city.
For example someone builds up an army with a navy on Britain and prepares an assault like on D-day. You can use ships and landing boats to attack/bombard the coast. The other player can defend with small pre-set defences based on where he invests his resources in, and has a base on the location to build and upgrade. If the attacker wins the zone he fights to another zone. Until he reaches the capitol of a country. He can chose to encircle and starve the enemy army out or push and capture the city. (maybe only 1 capitol per player, and other capitols with extra bonus when captured)
A capitol is a primary zone. The player needs to keep his zones connected when assaulting, if an enemy takes over a zone in an attack and there is no connection with the zone of your army you slowly lose starting resources and units. This way you can have a Schlieffen plan Assault to break a siege. Or push an opponent back like at Dunkirk.
Other World map possibilities:
War plan Red - Americans attack Britain
Operation Sea Lion - German invasion off Britain.