I think the next enemies would still be the 10 Commandments cause no confirmation they are dead yet. Then after dealing with them (either outright destroying/sealing them or making them (temporary) allies) we will face the Goddesses. Or it can be a 3-way war Not sure what role the Demon King will have though.
It seems now only Diane and Ban are behind in terms of Combat Class. But well, Ban can always go Festival Hunt so only Diane left. Her fighting style is quite physical so I don't think she can be one of those who has low reading but hax enough to fight much higher-combat-class fighters. And no one is gonna wait for her to dance long enough to gather power.
To think a pair of wings can increase King's Combat Class 4 times over...holy cow...
Meliodas and Merlin are the best drinkers? Wow. People from 3000 years ago are monsters LOL I actually like MelxMer.