A couple more ideas:

Merging all Townes
I’ve always thought it was cool how the whole world connected, I’m not sure if it’s possible now but it would be pretty cool if they connected more seamlessly without the load screens and portals.

Almost like an open world area but very few enemies since only a couple of the roads between Townes spawn enemies. But it would be cool to expand on the spawns though, like rare spawning world bosses.

Guild/Clan enhancements
Guild Bankwhere you could deposit an item for other members to use, you could set a password for each item or allow a certain member rank to access it

Guild levels
Level up a guild through activity points gathered by all members.
Points could be accumulated by:
-Killing a pve enemy or pvp opponent
-Crafting an item
-Selling an item in the CS
-Checking in for the daily blessing
-Completing a daily quest

Perhaps you could select a guild type when you create them to give bonus for certain tasks and reduce points from the rest. To make it fair if it’s a merchant guild or pvp or just leave it default if you plan to do everything.
Leveling the guild could unlock guild hall decorations, special quests, special items, special crafting recipes, or special vendors(maybe sell old crafting ingredients from previous caps).It could even unlock new maps just for the guild.
This would also allow for casual events of double guild activity points or something similar.

Guild Wars/Battles
There are tons of ways this could be done, my favorite idea I came up with would be to turn the guild hall into a pvp area where all members of the guild would automatically be on the same team. Obviously non-members can’t get in to other guilds’ halls, so to attack the guild hall you would have to get a key that would rarely drop by killing someone in pvp (maybe sell the keys for plat too)I don’t know if would be possible to select the guild hall you attack or if it should just be random. Once someone enters your guild hall with a key a guild message would alert members their guild is under attack so they can go defend it, but I think you should also be able to buy NPC’s to defend your guild even if no one is online.
As far as the reward, maybe an achievement and titles for “overthrowing” a certain number of guilds or a unique item or quest etc.