I've been playing Diablo III again, so my mind is saturated with thoughts of randomly generated levels, item crafting and base camps from which players strike out into the surrounding environs.
I'm thinking a combination of those elements with the open-sandbox exploration and construction of Minecraft.
You start out in the middle of nowhere with a very basic campsite. You have to start digging in order to find stuff you can sell to make money to buy tools and food and supplies and stuff. Nothing is useless. Even the stone rubble produced by sinking mine shafts can be used in a number of ways. As you dig and search for stuff to sell you'll find preexisting caves and tunnels, often inhabited by creatures of varying levels of dangerousness. You'll also find randomly generated dungeon levels of various sorts, like an ancient temple complex that sank beneath the earth thousands of years ago, or a subterranean jungle inhabited by prehistoric monsters. The deeper you go, the more dangerous the stuff found. You can use the money made from selling stuff and the resources you find to expand the camp site, increasing the sorts of things you can do on your expeditions.
Single and Multiplayer options. You could do an entire map by yourself, but the best randomly generated stuff is on harder difficulties, and on games with more players. While technically capable of doing everything, players would choose a specific class package determining what they happen to be actually good at.
PROSPECTOR: A general utility class, the best one for solo play, but somewhat useful in multiplayer as well. The best class at digging, their hardy nature means that they can take some punishment as well as fight back decently, but they're still out done in dealing damage by other classes. Also, their Jack-Of-All-Trades nature means they can't use the various Camp Upgrades to their fullest potential the way more specialized classes can. Their one shining feature in Multiplayer is that they are the sole class capable of Surveying, allowing them to determine what kinds of resources you're likely to find in a given area, as well as the ability to detect mineral veins to home in on while digging. Other player classes have to dig blindly, and have to go out and look to determine what's in the area.
SCHOLAR: The brainy sort, Scholars can't take all that many hits in combat, but have many other ways to shine. Their highly observational nature makes them adept at locating things like hidden doors or disguised booby traps. They can also attempt to identify items found while exploring, which includes such things as shiny stones and strange plant life. When forced to take part in battle, Scholars prefer to hide behind players with sturdier constitutions, taking the time to study the opponent and find exploitable weaknesses. Back in camp, they can use magical tools to prepare an array of special abilities to provide aid in combat, granting them the ability to stun, disorient, or otherwise negatively impact their enemies' threat level. In short, they are a sort of generalist magical class, with a bit of rogue thrown in.
TRACKERS: Excelling in ranged combat and quite good at the closer kind too, Trackers are usually the first into unexplored areas, where they use their keen senses to locate the creatures lurking about, even if they don't actually know what the creature is beyond "really big, and really toothy." They can also craft and use traps and snares to capture (or at least, temporarily detain) creatures. While keen eyed and well versed in shooting things, Trackers are not the tankiest people out there.
MERCENARY: The only purely combat-oriented class in the game, Mercenaries excel at weathering damage and delivering it. Armed for both close and long range combat, they're better at the first than anyone else, but can't shoot at the same range as the Tracker can. Their ranged attacks can hit more than one target at a time though, and they can make different kinds of grenades and other tools to use in combat. Outside of combat, however, they are completely useless. They can't mine, refine, build, or produce worth a damn, making them the hardest class to solo a game with. On multiplayer maps with a lot of action on them though, they are invaluable.
BARBER: Similar to the Scholar in that they both prefer to inflict status effects upon enemies, Barbers differ in that their skills are more defensive in nature, meant for reducing the amount of damage dealt by opponents. They can also provide medical care for players, setting broken limbs and treating infected wounds. They don't possess the Scholar's academic or exploratory skills, however.
TRAINER: Capable of training animals found on expeditions, they can also provide a bit of healing, though not to the level of the Barber. In combat they direct their animals while occasionally doing some damage themselves, but without a pet to do the tanking and most of the DPS they're helpless. Outside of combat, they can train additional animals to help out at the Camp or in the mines, though they have to keep track of each animal's health and status to make sure they don't act up due to hunger or disease.
BUILDER: The opposite of the Mercenary in that this is probably the least combat orientated class in the game. Though strong and possessing great stamina, Builders possess no special abilities geared toward fighting, unlike the Prospector who can fight to a certain degree. The main job of a Builder is, naturally, building things. Specifically, a Builder's job is to refine raw materials, build and repair structures, build and repair Camp Upgrades, and enhance mine shafts and caves that have been cleaned out, turning them into useable spaces. While a crucial support class, it's not very glamorous, and more adventuresome players should probably pick something else.
MERCHANT: The second of the non-combative classes, Merchants have a strong supporting role to play. Better at buying and selling than the other classes, they also manage the camp funds better. The second class to focus heavily on item crafting, Merchants differ from Builders in what they can make. Unlike the utilitarian and industrial tools and devices of a Builder, Merchants produce mainly personal items. Like the Scholar, Merchants are very adept at creating and using magical essences, and can apply them to a wide variety of objects. Finally, Merchants function in a managerial role, capable of keeping track of the condition of the other players and arranging things at Camp so that everything goes smoothly.