Some ideas for inspiring:
-Could be factions, like faction A, faction B and neutral faction.
Here we can have two ways of implementing this:
You could choose a faction (every sunday maybe and you will have the whole day to choose) but the factions always should be balanced in number of guilds so you will not have an unbalanced number of factions out there.
The guilds who doesn’t choose a side will not participate at all and not receive rewards in case that we have rewards.
Another one is to have the factions random assigned with a nice matchmaking system so the two factions will be balanced between them based on a good algorithm and ranking.
-The killing event could be only a few days a week instead of every one hour, or maybe between 3 hours so every 3 hours you will have 1 hour that you could kill everyone(or whatever number that makes ppl happy)
-Could be have some special GvG events or quests.
-Rank system, every time you kill players of other faction or things like that(doing quests) and based also in whole gold of the guild/whole rb/whole hours played) you could have a rank in game like a leaderboard, later on the first or three first guilds of every faction will have some reward
-Implement some in game system to manage guilds better, like searching, recruiting open and requirements to have open entry to apply to guilds etc like any other game out there, could be with some GUI or with an NPC in city like the broker/market, also promote, demote, transfer ownership, etc. Take ideas from Clash royale or Clash of clans
-Guild bonuses like better drop chance or <insert something here to win/earn> to apply to the guilds winners(nothing about gold and exp please) this will increase the bridge between noob and pro guilds
-Lower the guilds to 15 or 10 people, 20 is too much.