Righto.
Well, for the online play, it would have some towns that no one can attack each other in(for those who do not want PvP in online combat), a town or two where you CAN attack each other, but only if they're on the opposite spectrum(such as good can only attack evil, etc... and it would further contribute to their alignment), and a town or two where everyone is fair game. However, the fines and power of the guards would change as each town eases up on combat restrictions. Hell, there would even be some fields where you could do large-scale battles as NPCs in the area register you to one side or another. Battles like these will range from regular skirmishes to a battle between the forces of Good and Evil!
CASUAL PLAY
For those who just like casual play and stuff, you can merely go to random dungeons or do quests in order to gain certain benefits, such as extra weapons or armor/clothing pieces that you would never be able to obtain in Offline Mode. Rather simple, some of the more powerful, cooler-looking items (Such as a dark version of Archon's Armor or armor pieces of main characters) will require a friend or two to help. Nevertheless, far worth it. You would also be able to go to a town alchemist and have your gear custom-colored in order to suit your tastes for a small price. NOTE: Any items you obtain in Online Mode, you WILL be able to bring with you in your Offline Profile!
If you want more stuff for casual play, there is always being able to team up in an Arena to combat the hordes of monsters they throw your way... or perhaps one of the many instanced dungeons that lay scattered across the land, just waiting for you to explore it and uncover its treasures.
Also, like I said before, you'd be able to create Guild Towns, and actually hire NPCs to set up shop to sell you nice goodies for when you're heading out, AS WELL as hiring guards to protect your town(you cannot hire mobs like Balverines or whatnot, but we'll get to that in a minute). How would one afford this, though?
Via donating in the "Guild Bank". Every fee that relates to the city would be deducted from the Guild Bank... and while the fee grows as your guild town does, the fee would still be wholly cheap, so make sure casual guilds could still afford it easily.
BEYOND CASUAL: HARD PLAY
"What if I don't WANT squishy humans as my guards?"
Then a special, rather difficult quest comes about (think about 10 people needed just to live), in which you can infiltrate a hovel of a certain group, and if you beat the crap out of enough of them... you could actually FORCE them, into submission, to act as the guardsmen of your town! The number of guards present at any time would depend on the type of guard you have picked. Mob guards would be limited to Balverines, Bandits, Hobbes, Fairies, and Minions. NOTE: Mobs will be noticably stronger as Guild Guards!
For those who want a LOT more bang for their buck in Guilds and are ready to become more than casual... suit up and saddle up, 'cause you're gonna need a Power Crystal.
Power Crystals will be at the very heart of your Guild, providing a VERY interesting bonus: Ability to warp to the entrance of Dungeons. This includes dungeons that would normally have you traveling at LEAST 15 minutes to walk to. It also allows you a Health and Mana regen bonus while you're in your town. An obviously coveted item(though not required at all, making it an optional deal for anyone, and something casual guilds actually might want to avoid), which your guild must earn in a massive quest which is not for the faint-hearted. NOTE: You may only have one Crystal at a time!
...and while recieving this Crystal would give guilds a nice boost, it also flags them to the Guild NPC, informing others that "The Guild of [insert name] is becoming too strong! I beckon you, gather forces and please slay them!" This would be where guilds can raid other guilds.
The guild in question would be able to set up window slots so people can't just waltz in and take it... a 3-hour time slot assigned to two days of the week, and you can only stay in their guild territory for one hour. This also lets the guild prepare itself.
During this raid, a guild may be able to take many things, but not EVERYTHING. For instance, you can't raid a guild bank down completely. You can only take about 1/4th of what's in there at the moment. Also, while you can raid NPCs of the items they sell, you would NOT be able to enter personal homes and steal their stuff. That'd just be mean.
If the guild that was raiding managed to reach the Power Crystal though... they must overcome an enemy/enemies that will be lying in wait, protecting this crystal. It may range from a few Trolls to other smaller bosses. NOTE: This is the ONLY occassion bosses will be on your side. Boss types will be dependent on your guild environment. They will be slightly more powerful to combat multiple players!
The aim of this raid would be to SMASH the Power Crystal, and collect its shards to bring back to your guild. However, players may still be able to kill you to reclaim their shards if you lollygag around enough. Should the invading guild ESCAPE with the shards, they will need to pay a hefty fee to the town alchemists in order to piece together this shard... even so, it will take 3 full Earth Days to complete it. Once completed, all the above rules immediately apply. This also means the guild who had previously owned the Crystal will not be raidable anymore, until they either get their Crystal back, or obtain another one.