Admittedly, Mojang isn't a blockbuster studio, but how about Bethesda Softworks, the company behind award-winning games like Fallout 3? Their community has a huge following of modders, no game moreso than the aforementioned Oblivion. Oblivion's community had so many complaints regarding the game's design that players decided to take it upon themselves to rectify the problems. This led to a veritable plethora of Oblivion mods that "fixed" things that gamers thought were broken. (For reference, The Elder Scrolls Nexus, a compendium of mods for Elder Scrolls games, lists 25,235 files in their database. That's a lot of mods!)
One mod in particular, Oscuro's Oblivion Overhaul, was incredibly ambitious and basically redesigned Oblivion from the ground up. As the website states, "It affects almost all aspects of the original game: Quests, Environments, Dungeons, NPCs, Creatures ..." The list goes on. This mod was so well-received that Jorge "Oscuro" Salgado became one of the developers on Fallout: New Vegas.
But maybe you're not interested in modifying existing games. Maybe you want to create your own title to earn your way into the industry. Well, if that's the case, you're in luck, because there are plenty of opportunities to do just that.
Every few years, Epic Games puts on the Make Something Unreal competition. The premise of the contest is simple: Teams of modders utilize the current Unreal Engine to make total conversion mods (standalone original titles based off a game engine) that are judged by various professionals in the industry. To date, there have only been two Make Something Unreal competitions - one in 2004 and one in 2010 - yet the competition quickly gained quite a deal of publicity as a place for fledgling developers to make a name for themselves. The winning team earns a cash reward as well as a license for the current Unreal Engine. This amazing prize makes it possible for prospective developers to form their own studio.
The winners of the 2010 competition were an unnamed team and, according to their site, their project The Haunted is quickly on its way to a full release. Additionally, Tripwire Interactive, the first place winner of the 2004 competition, has already proven itself to be a successful studio. In 2006, they developed their award-winning mod Red Orchestra into a standalone release, followed by Killing Floor. Like Red Orchestra, both Killing Floor and The Ball (the second-place winner of the 2010 Make Something Unreal competition that was published by Tripwire), started as Unreal Engine mods before Tripwire released them as full titles.