Even with Grand Theft Auto V being one of the best selling titles in recent memory, Rockstar is likely to move on at some point down the line. Judging from a proposed actor CV clip, work on Grand Theft Auto VI could have been going ahead for some time already.
NeoGaf user Chadskin noticed a resume up on Breakdown Express listing non-union actor Tim Neff as having done motion capture work for next year's Red Dead Redemption 2 and Grand Theft Auto VI.
It seems like a major oversight for a person to list an unannounced game on their resume, but there's likely a multitude of reasons it could end up on there. If it's a promising gig, of course they'd want to see potential employers see it at a glace. Sometimes those brought in to work on video games aren't too clued up on the secrecy of the industry. In this case, it seems plausible to imagine that if Tim Neff did indeed lend a hand in Red Dead Redemption 2, Rockstar would have called him back in for their next project. Maybe they just ran both gigs simultaneously to save time. We can never really be sure.
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Either way, given Grand Theft Auto V released back in 2013, it really is time to stop banking on Rockstar pulling through with that single-player DLC. Teased numerous times and even heavily expected given the track-record of Grand Theft Auto IV, it seemed Rockstar scrapped the idea instead focusing their efforts on Grand Theft Auto Online. It's a money machine.
There's going to be no real way of confirming this as false or fact, so we'll just have to use our imagination. There's absolutely no reason why Grand Theft Auto VI would never be a thing, but does Tim Neff already know far more about it than we do?