As you can probably guess, there are several large spoilers below for the DLC. If you want to save yourself some surprises, stop reading now.
First up, he's uncovered what he believes is the DLC's official blurb, which refers to a new swamp/jungle continent called Aegrus. It reads, "Danger! Excitement! Mustaches! It's time for another episode of Vault Hunter Adventures, featuring Sir Hammerlock! In this week's tale, our hard-boiled heroes travel to the savage continent of Aegrus! Their goal? To uncover the most exotic creatures Pandora has to offer, and give 'em the old one-two!"
Each class will also reportedly get a new skin and head, while a new vehicle called the fanboat will also be added replete with 68 skins and corrosive, incendiary, and shock weapon mods. 12 side missions and 5 longer main story missions will be included as part of the package, as well as a new raid boss (supposedly a Stalker the size of a small building), seraph items and enemies. Sir Hammerlock himself will be providing monster-tracking side missions, while "a self-important but not-so-well-known new enemy" will also play a key role.
That's all for the Big Game Hunt DLC, which is codenamed "Sage", but MikeTheInfidel has also uncovered details pertaining to another future update called "Gardinia". The main thrust of this update promises to be character customisation, with each of the five current classes getting "three customisation packs containing a new head and a new skin, for a total of 30 new customisations."
There's been no official word on when Sir Hammerlock's Big Game Hunt will release, though the logo for the DLC appeared to leak late last week. When IGN reached out to 2K for information on that, we were told "There are still two add-on campaigns scheduled to release as part of the Borderlands 2 Season Pass and we are really looking forward to sharing news about our third add-on campaign and other downloadable content soon."