Guild vs Guild map
Or make it bigger and better with the following:
Make a open world map that link single maps to eachother with portals with other Guilds on them. One map consists of 3 guild. That is one alliance. So they can battle against eachother. On the map a Guild can own a small outpost. They can upgrade the outpost to small keeps until one big fortified castle!
Make people mine for stone/wood for building walls, iron for gates/canons etc.
maybe some special resources that is harder to find that your guild need to gather serveral maps further. (escort the gatherers) Build/Create guards, patrols, several strenghts.
So many idea’s and ways to make this kind of GvG interesting. Maybe a whole economic system can be created.
Maybe resources can be gathered in the pve world and put into the guild GvGbank.