May I introduce you to Ultima Online?

You have some items that are not lost upon death (either by them being special and "blessed" or by you paying some in game gold to insure them (costing you every time you die), but everything else? On your corpse, ready to be taken by the nearest player and even humanoid enemy. More than once have I seen a lich just grab some of my stuff and cackle off back into the dungeon.

Aside from being able to lose stuff on death, there are actual Thievery skills ranging from sneaking to pickpocketing and lockpicking (for stealing from player homes or when you unearth treasure chests). There is a "safe overworld" but many popular private servers have removed that. You might go about your daily business to grab some stuff from the bank teller and next thing you know, a grandmaster thief took your precious sword from your backpack (there are a lot of skill checks involved depending on how many possible witnesses there are (player and NPC), if the thief is invisible, how heavy the item is and such).

Very tricky to accomplish as long as people can interact with each other in the real world and bypass whatever rules and restrictions you have in the game rules, be it in terms of just communicating in situations where they shouldn't be able to or running multiple accounts and characters at the same time to boost their power and influence.

You'd need to get extremely draconian with the anti-cheat, require real world ID for account creation, not allow new player once the game started and all that.