As long as you make the experience fun and engaging anything is possible. I think, making PVP open world and cut throat is a ballsy move that if you can balance it well and have appropriate consequences is perfectly acceptable. A bounty hunting and criminal system is a grand way of doing things, especially if you allow players to get into the mix as the bounty hunters.
However, a major consideration is how you approach your player level system in relation to the world. Open PVP works fine if either A) the world map properly segregates players by level and prevents, or at least limits, the potential for negative play of someone max level coming and picking on new players or B) in an open world sandbox style game levels are not a direct representation of strength but a reflection of options. In B's sense, its not that you hit for 1 and I hit for 100 but I have 100 ways to hit you for 1. In other words, you don't auto lose, I just have more options.
If you intend to have a freely roamable open world where players of all levels can interact, then there must be something in place to keep a high level player from harassing a low level player. Or at least make them think twice about it.
Also, making sure players understand they are going into potentially hostile environments is a must. Nothing creates a more negative experience than thinking you were cheated or your lose was completely out of you control.
I think having placeable guards near controlled territory or settlements as a way to create a safe place for your denizens would be a great idea, too.
I think an interesting idea would be to look at what games like City of Heroes or Star Wars Galaxies did with their player generated content systems. I'm particularly fond of SWG since it encourages people to play the game itself to be able to build quests. Taking it even farther in the future, incorporating player kingdoms or settlements and their surrounding lands into it so that they act is their own mini theme parks of player generated content and modules. That can act as a way to breath life into the settlements and even allow different player towns to interact. Imagine putting a quest NPC that you've made into your own town, then with the help of someone in another town, there is another NPC you go to and continue the quest, etc etc.
Getting things to work with the larger over wold might present issues, but having quest specific personal instanced dungeons and areas might be a solution, or putting in specific nodes that players can use for their quests. Say I build a quest and need a cave or house, I find one that has been put in the world as a usable node and it would create a customized instance when someone on my quest accessed but still be usable to anyone else for their own quests, but to a passer by or someone without a quest that used that node, it would just be a piece of nice scenery.
It presents some interesting and exciting possibilities.
Its a fair bit like how SWG does it, but I think Pathfinder presents a excellent way to deliver the content to players too. You could have Pathfinder Society guild leader NPC that would essentially act as a merchant selling/distributing the quests in the form of letters. A player chooses a particular one, opens it, and the quest is presented to them to start. Add in the ability to invite other players to participate in a quest you've chosen and I think you've got something that not only represents player modules and the Pathfinder Society, but also creates endless, player voted on, content.
I don't like the idea of real time combat in a potentially lag heavy space. Its imperative for that type of gameplay to be fluid and precise, something you simply can't get online, not perfectly at least.
That said, I want something more active than the simple auto-attack and press specials as much as possible. I think Wrath of Heroes, TOR, and other MOBA games like LoL are on the right track in removing an auto-attack but keeping a lot of the more heavy RPG percent elements effecting damage and hit chance, etc.
Honestly, I think that class based isn't the way to go in a Sandbox style game, especially one trying to adhere to a RPG mentality of creating your character. Class based was created as a way to simplify design, but ultimately it limits player choice. At this point, I feel that the class based model has run its course, it has be tried and tested into the ground and there's little more that can be done with it. These days, class based games are trying to make things more flexible and allow players to break out of their predefined, game-mandated roles and branch out into whatever the player wants to do. Freedom of choice should be at the forefront during design.
Admittedly, skill based is harder on the designers in a competitive multiplayer environment, there's a whole lot more to worry about both in terms of keeping over lap of skills to a minimum and making sure everything is balanced so that building a character is left more to how the player wants to play, not what plays better.
On top of that, skill based is something that hasn't been done a whole lot. A lot of the bigger MMOs adhere to class based but that creates a golden opportunity to capitalize on something that is severely lacking in the market and do so with a property that lends itself to it.
The real montra of sandbox style play is 'Here are tools, go do what you want to.' With class based you aren't giving a player tools to play, but instead telling them how to play.