I still don't see the need for a change of the system.
But if that's such a big issue for you, maybe you should change your recruitment strategy.
If you don't want any PvPers or thieves in your ranks then check the people you join on beforehand. Let them enable their myuo skills view and look at their skills before adding them.
Sure noone will prevent that person to gain PvP or thievery skills afterwards but that would be up to you to check.
To get back to your actual idea to add an option to the guild creation where you can choose to have PvP or not. It would have a big impact on the game mechanics like preventing you from entering PvP rulset zones. Also you will not be able to switch that after guildcreation. Elsewise it would lead to much more "abuse" than before.
Also, if this option was there, you would have reduced a guild to the tag shown. There would be no benifit to other players joining besides them carrying your guildtag. I really doubt anyone would stay in such a guild in long term. Well, some hardcore crafters maybe
Also I can't see any what's wrong with that. Joining a guild means that you can attack each other, which is good for training purposes and rich interaction experience. If you don't want that don't join a guild. You could still organize a guild through a homepage without using the guildsystem itself. I see it that way. Reducing a guild to showing the guildtag makes it obsolete. You can accomplish that using a homepage and icq as well.
Manageing a guild has never been easy and will never be in no matter what game. Or do you want to tell me that those kind of people who would steal from your guildmates are people you want to keep? You are comparing with real life a lot. So let me say, if I had someone in my circle who steals, no matter whether he can do or not, I would not want to keep him.
Als in regards to stealing, we apply the following rule for example:
Thieves and Stealing
As well as the contents of a yellow bag the following items may be stolen: Any amount of gold, gems, bandages, petals and common potions, books, food, drink, non-magical jewellery, arrows, bolts and any amount of magery and necromancy reagents.
Anyone who joins needs to agress to that. Being caught doing otherwise will result in warning or booting from the community. So everyone in our ranks DOES know the DOs and DON'Ts on beforehand. They do not need to join if they don't like it that way. It's that simple

It works same way in real life. You get employed and it is forbidden to steal stuff from your company. If you do, you get fired. Exactly the same.
If you want to lead a guild, then you need to do something for it. Be it organizational or whatnot. I don't think anyone would agree implmenting a sissy Guild feature

Guilds have always meant to enter competition. That's what they are made for.
In regards to your comment about making others angry. We are caring for our members across the (I can't oversee how many guilds) community. And our GMs are dealing complaints seriously and professional. If it would mean angrying one single person because that one broke the rules, it's fine because it will make x others happy.
To get to the point. If someone joins your guild to steal from your members, then that person would not join for other reasons anyways and sure not contribute to your guilds' growth.
I do see that you would like to have this sort of "be safe" option on your guild creation. Besides turning off a lot of fun with that I doubt the afford to implement it is worth the effect because you can accomplish things otherwise.
Knowing all that, do you really want to waste the Dev's time and money for things like that just because you got an organizational issue? Their money is much wiser spent for fixing bugs and other stuff.
And if you don't like how uo "is" then it might be not the right game for you anyways. Would be sad to loose a player because of social issue though.