In Multiplayer in could be used in many ways. Maybe it could be set by the host.
For fairness i would just save the Map progression what country belongs to whom.
1.) One of it being a 24h Server where you join and play for pure fun with others until the Map is reset or completly conquered.
This also acts like a bigger social thing. Such a server would need atleast one human player for the fight to continue. Then also the AI can choose countries to attack.
2.) Second usage could be for smaller Groups that gather on predetermined timeframes and play one or two maps and pick up with their progress the next time they gather. Mostly focused for smaller LAN/COOP Groups.
3.) would be a pick a map and play option like it is currently for quick games.
I thought that all maps that would be made for some kind of world map would also perfectly work for any kind of quick battle.
This game has the major advantage that it's "nearly" nothing that changes. Only things that would be added in a Metagame would be stuff like Air Support or some kind of Unit Order and delivery mechanic if you own a country that belongs to you and has some sort of Airfield, Tank Factory, ....
Only decision here would be if those production buildings would be capturable while playing such a map or if those serve only as map modifier without being represented as buildings. Hope you understand what i mean.
There are also a couple of mechanics how you would like to handle reinforcements. Either through a cooldown, a limited amount with the maximum usage being the amount of factories/airfields you hold in all your countries or a resource system where you "buy" airstrikes or vehicle delivery. Resources could be gathered per soldier kill or per country you hold after a "turn". It could also be a mix of everything together.
The Metagame is turn based where every team chooses after a match what map is going to be played. First one team and after the fight the other team get's the choice to choose.
I would add a save feature that after you are finished with a map the progress of country possession is saved. If due to time constrains a map fight isn't finished then the progress is like bevor the fight. Atleast a minimum of 1 human player is needed to count the map fight into the overall progress.
The host could also get the option to let players join a side as they wish or that it should be automaticly be evened out.
With a good system you would have the ability to go any way you like. I think that it adds even more "war" feeling that can expand any way you wish without changing the super cool core gameplay this game allready has.
It's true that Maps can take a decent amount of time to play. Clans maybe will only choose the option to play a single map, others will rather choose the Worldmap option. Either way it's a choice. I would see it as additional customization option for the game to go the way you like. Every map than can be played in such a Metagame would still be fully functional and usable in a Quick fight for clans or people with the desire to play just one map because the maps themselves won't change in any way. It's just the modifiers in a Worldmap campaign and the Area progression that isn't there. Nothing is really lost or nerved or anything.
Maps that are made for a Worldmap also can range from rather small to verly large where those people who like to fight Quickbattles get even the option to play on either a small or large map depending on how much time they have. Also the Maps would look very unique because you maybe would build a city map or a river map with bridge or one that looks maybe like a second omaha beach.
If you'd like you could even let both sides start with a fixed amount of maximum units and increase the maximumm slightly for every map that is captured that has/is a city. Just a little bit of idea spreading

It's also your choice if you'd make desert maps, island maps, jungle maps. Or even add water based transport/battleship artillery units...there are many many possibilities.
That's why i came up with this idea in the first place. I tried to imagine some kind of overlay that changes nothing and let's everything fully funtional as it is, while giving another option that is fun to play and can be used for many ideas and has the potential to pull in a bigger playerbase because of more options that could satisfy many tastes.