That is if there ever be something like “Guild War”…
The “war” itself is based on invading / defending guild bases and these bases are all around the world. In the basic concept, this works similar to some other games which I think most of us here already know, so firstly I will jump into the reward part.
Rewards
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When you successful capture a base you also “own” its surrendering (fields, dungeons, etc).
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Guild members on maps belonged to their guild will get some benefits like buffs or bonus EXP and maybe drop-rate (a little, enough to make them feel rewarded somehow and not too much to break the game).
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Some hidden maps on the arena will be unlocked for the guild like a certain MMORPG game. These maps have special monsters with their special drops. The guild master can choose to open the maps for other players outside of his guild but they will have to pay to get in, this will be handled automatically by the game.
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A big place like a hideout with social aspects for the guild. It’s like the barrack but for every and only guild member. This is for the sake of hanging out with your fellow guild mates and role-playing! Maybe the hideout can give some benefits like having a save point, better crafting stations, etc.
With this people can feel like interacting more with the environments and that they actually “own” the places they fought so hard for. It’s more fun and more closer to role-playing than just get rewarded with some items. Also it may actually help making players leveling, farming on a more variety of maps based on what places their guilds own.
Characteristics For Guilds
Guilds will try to take over places that are more suited for them. For example, a guild which is made mostly of Druids would want to take over an arena with lots of grasses and a druid would prefer joining a guild like that. This gives more characteristics to guilds and make the role-playing part better. Of course there would be arenas which are more suited for guilds with more mixed kinds of members and those arenas should be the majority because that was the main point of having lots of classes and letting people grouping together.
But I admit that guilds of Paladins may want to take over arena with lots of demons, ghosts, undead or guilds of Cryomancers want places filled of fire monsters… This will be rather odd… We can make this less worse by giving buffs that are related to the characteristics of the arenas. Like snowy place would give a buff for ice damage, then those cryomancers might want to own a snowy mountain instead.
Battlefield Environment
The environments of battlefields during a guild war should affect the game-play, the battles in a way that reflects its characteristic. A simple example is a battle field on a grass land of course would have lots of grasses (it’s fine, I already said “no sh!t” to myself, you don’t need to), or a castle inside a volcano will give some buff to fire damage, etc. So a guild with many Pyromancers will more likely to own that fire castle. First it makes the game-play more deep, next it put things into interesting settings! Deep in a volcano surrounded by magmas is a hideout of fire users! And that creepy place filled of corpses is belonged to the ones who control the deaths!
Maybe it also gives debuffs to its very owners like the place of corpses will make its owner more vulnerable to holy element.
Guild skills might affect these effects.
Another Choice For Reward
Maybe you successfully invaded a base but don’t want to own the place and the rewards based on its surrendering, then you can choose to just get some items instead. This also put things into settings, like the place of necromancers we just talked about, it makes their owners more vulnerable to holy element so an army of paladins would love to use that advantage to crush these nasty corpse lovers. We will have this setting, an army of paladins exorcised a guild of necromancers, temporarily gave peace back to the place then they left (with their rewards, of course).
This Is Not Unfair
You might feel that the rewarding system is unfair to players who don’t join a guild but I have to say that fighting in a war between guilds and coming out as a winner is pretty costly. First it costs your time, time to fight, time for preparation. More than that it costs your money, your items, you can use a whole lot of potions and have all of your equipments broken and get absolute nothing at the end. So it’s fair at least on the concept, it’s fair for people who do not join guilds and do not attend these guild wars. It might also be more fair for the members of the winner guilds because everyone get similar benefits and don’t have to be depended on their guild masters for rewards.
So I’m fully aware that this will never happen, just put it here because it’s fun playing with ideas. 
Though I really wish there will be a guild war feature involving invasion mechanic.
Thank you for looking.