I imagine a conquerable territory, divided in regions with resources, quests and NPC trade cities.
Guilds can create an alliance and build control towers -> cities -> citadels to control each territory.
A net of controlled territories can give the sovereignty of a big region (kingdom) to the alliance the guilds are in, giving access to all the resources of that region.
Guilds have to conquer the net to make the territories first contested and then, after days, conquered and then take the region.
Alliance can spread the income of the kingdom to all the guilds, due to the importance of the guild owned territory (river, mountain, ecc.), the size of the guild army, ecc.
Guilds armies can siege castles for days, cutting off resources until the death of the sieged armies, or assaulting the remaining army with many losses. Sieged guilds can instead wait for allies’ help, or do a raid in the night to break the siege.
It would be good to see mercenary companies too, that fight for gold or a castle.