I just let my mind flow and wrote tons of ideas while having fun. Don't take it too serious!
Type: Guild Hosting
Suggestion: We have many buildings in Prontera/Alberta/Morroc/Izrud/Other fields. Let them all be buyable by guilds. Price depends on location, size, uniqueness and architecture. That way towns are getting more lively.
- To enter the bought building each player of a guild has a link to warp them always in whenever they want and whereever they are.
- It can also be entered directly at a building (NPC or Warp).
- Everyone (also people who are not in the guild) can enter guild's building for visit. Guild itself can decide whether they allow people in or not and does have an option to kick/ban people from their building or kinda a friendlist which are allowed to enter. Also an option for rooms to make private rooms for leader/manager or whatever.
- Each Guild can only buy 1 building. If they decide to buy a bigger one they are allowed to change it.
- Not letting a building going into ghost house inactivity for a certain time will throw the guild out. There also should be an option for guilds to give up their building and getting some zeny back they used for, like 20-30%.
- Or Tax System: Paying tax for using a building.
- Every building is independent from channel.
- Prices (just examples for fun): Little boxboard in a corner 100z, a tent in a camp in dayr desert 20'000 zeny, the huge building in Prontera where the AH NPC is in (except the room with AH) 10'000'000 zeny or more.
That way small guilds have a chance to grab small houses and huge guilds can focus on ernomous building, which should be payable by them.
- If you warp into the building it should keep it's shape and size, probably a bit more sized, just more realistic how it looks like. If the building looks like it has rooms, floors, garden, doors, windows...it also should be that way inside.
- Decoration: The more the better. Changing colors, arts, ground types, walls. Everything is possible if it keeps the shape of the building. You can also change the size of rooms.
- If a guild decides to change their building any decoration etc. they leave there or are not takeable (wall, colors, ground etc.) will be kept to next guild who gonna buy it.
- Getting a building is of course empty at beginning, just it's architecture is there. Getting more things or services in costs again a lot of zeny, kafra points or time clearing quests.
- Inside the building a guild can optain/buy kafras, storages, warp npc(?), mailbox, announcement board (not just 1 little sentence, should be able for some huge guides/events/explanation/information/rules etc.) and so on.
- Getting those NPC's also depends on building, some basic NPC's like kafras are everywhere obtainable.
- To make it more interesting for non-guildies: Merchant NPC's inside the building or outside in front of a building. Guild can decide what they sell and at what price each item.
- To make guild buildings unique, amount of merchant NPC's are depending on building's size and goes up to 10 (for example for the Prontera building i mentioned before) and there are tons of merchant NPC's you can choose: General, Food, Craft, Gears, Accs, Potions, Cards, Kafra Cash Shop Items, Mats, Decorations for building, runes, guild skill books, event items, any other crap/ideas.
- Logo: For example 500x200 image putable on a sign or flags at the building, for example next to door. Just showing people outside "Yes, we live here!".
- Some extras depending on building: For example the huge building in Prontera i mentioned before can offer a bank service for guild and non-guildies. Non-Guildies need to pay some fee decided by guild to use this service. The price should be limited not going too far up. That way guilds get a little income. Other Examples: Resting Area at camps, more Storage service, personal pet services for a limited area (looting pet, extra bag pet etc.), buff service, ship service at ports, Inn (let player sleep for fee and while logged off getting little amount of MP per hour max. 8 hour xD), Restaurant (Let player choose a meal and eat it for 10-30 minutes depending on meal, after finishing a meal it gives a buff for some hours xD), and so on.
- Sign In Boards: To leave a message for guild. Guild can decide to invite people who left a message via mail or direct invitation.
- Quest Board: For Guild Quests, which is activeable by guild leader. Once it's active any member of guild is part of this quest: For example Kill Mummy 10'000 times or Kill Osiris 500 times. If one of the guildies killed a monster needed for guild quest it automatically gets updated at quest board. Or obtaining a huge amount of an item which needs to be given to quest board to get updated. By finishing a quest guild can get for example zeny, guild exp, decorations, NPC's, services, special items etc.
- Thriump board: Just for showing off. For example how often they killed a boss, how often a member got 1st place in colo, how often they won a castle etc.
I for one would love to build my own dungeon then invite the rest of the server to try and beat it. 
