If you've already played Grand Theft Auto V on the PS3 or 360, you'll be eligible for a bunch of extra weapons, events and vehicles when the game re-releases this year/early next—even if you're going to be playing it on PC. Rockstar has just announced the "exclusive" content, which I'll detail below, while confirming that players will be able to pick up where they left off on the last-gen consoles, providing they're members of the Rockstar Social Club. That membership is also needed for the new, returning-player-only stuff, which includes a Hollywood murder mystery, photography challenges, and a hatchet.
Basically, returning players will encounter additional events in the world, which present the player with challenges that will unlock new vehicles, weapons and visual filters when completed. These include the aforementioned murder mystery—"As Michael, follow a trail of cryptic clues to unravel a grisly murder mystery. Put the clues together and you'll unlock two Noir Style Filters, which will give your Story Mode gameplay and Snapmatic photos the feel of a gritty old school crime classic"—a wildlife photography challenge that unlocks a submarine, stock car challenges, monkey graffiti and more. Unlockable extra content includes the Imponte Duke O'Death car (pictured above), along with the rail gun and hatchet weapons, muscle cars, and the "faster and more maneuverable" Xero Blimp.
As mentioned before, you'll need to have played the game/be about to play the game on PS3 or 360 to get this extra stuff, and you'll need a Rockstar Social Club account to make the transfer to the new platform of your choosing (additionally, the save data transfer can only be made once). While the PS4 and Xbox One versions are due this November, the PC game isn't expected until January, so we still have plenty of time to have a go on a last-gen version to claim this extra content. It's not clear whether this transfer scheme has an expiry date, but hopefully all this extra content will be made available to non-returning GTAV players at some point.
More details here (opens in new tab).