Based on the info provided by TezFunz2 and Yan2295, the update will be adding a new mechanic similar to being a VIP or CEO. This means that membership in a biker gang will be a similar position that will have some entry pre-requisites and will differ from regular play. This is suggested by various new mission types being prefaced by "biker", which is in turn prefaced by "gb". The gb code has been used to denote VIP and CEO missions in the past.
The mission types, based on the code, will be Joust, Initiation, Rescue Contact and Bad Deal. Chances are Initiation missions will be one-time jobs played upon first becoming the member of a biker gang. Rescue Contact and Bad Deal seem straightforward enough, however depending on how Rockstar is interpreting Jousting on bikes, that could be the most interesting of the four.
Another mission type with the "gb" preface (a specific type of reference found in the files), but lacking the "biker" denotation is Vehicle Export, which might be a new CEO challenge or job. This application of the CEO mechanic to biker gangs is a pretty novel idea, and not one we would have expected.
Another interesting entry is "am_mp_personal_mod_garage". Now, "am_mp" signifies player properties in the game, including apartments and garages. With the release of Cunning Stunts adding 13 vehicles with another 3 having been added since, players have been clamoring for more garage space even more than before.
This entry seems to indicate that there will be a garage expansion coming with the biker DLC and that the new garage will be customizable much like the interiors of CEO offices are now. This new novelty is bound to please several fans, however it raises the question — if the 13+3 vehicles of Cunning Stunts weren't enough to get us an additional garage, how many bikes will this DLC add?
TezFunz2 has also verified some other leaks regarding the Stunt Race Creator that will soon launch. Some players have managed to access some of the new props and features of the creator. Obviously all the props used to make the official Stunt Races are in there, however some new info has also been revealed.
One of the new features will be the ability to remove default world props while in Creator mode. Basically, in Creator currently, you build upon the default GTA 5 world map. You can add props and delete your newly added props, but you couldn't edit the actual game world. This will change with the release of the Stunt Race Creator. Another interesting addition is the new speaker prop. When placed, this will allow you to trigger specific sound-effects or music in close proximity to it.
Unrelated to the Biker DLC is another interesting tidbit of info. We've known since the leak of the additional Cunning Stunts vehicles that there will be at least 2 mini-updates between the release of that DLC and the Stunt Race Creator. Now it seems that the second of those will contain a new Adversary Mode. In spite of the stigma Adversary Modes have gained over the months, the recent success of Power Play might put this one into new light as well.
The leak indicates that the new Mode will be called Entourage and the info has apparently come from IGN. The mode will be an asymmetrical setup with a team of assassins going after a VIP who is being escorted to checkpoints by a team of bodyguards. The goal of the assassins is to kill the VIP as quickly as possible, while the bodyguards must protect the VIP while he/she reaches all the checkpoints.
Which of these new GTA Online leaks excites you the most?