We don't know a great deal about what's next for the Grand Theft Auto franchise - the fact that there will be a GTA 6 at all is only known from CV missteps by former Rockstar employees and the minimal deduction required to know that a brand this powerful is obviously getting more games in the future.
One particularly studious fan, however, might have uncovered the next protagonist - and it's been teased for the past two games.
If the name Sammy Bottino doesn't immediately ring a bell, you'll be excused. He isn't a protagonist like Tommy Vercetti or Franklin Clinton, nor a major character like Roman Bellic or Ron Jakowski. In fact, he hasn't ever appeared personally in any GTA media to date. However, his name has been mentioned a lot of times in important contexts and other key events link him to the storylines of the games.
Sammy Bottino is a skilled gangster and dangerous criminal - sort of like every GTA protagonist so far - who was at some point the right-hand man of Jon Gravelli, leader of the Gambatti Crime Family from Grand Theft Auto 4. Throughout the game, Niko and thus the player, learns some additional key information about him.
Police Records indicate that Bottino was very active in the 80's and 90's, with Grand Theft Auto (hah), hijacking, manslaughter and racketeering all on his record. We also learn he was born in 1965. He also gained notoriety in Vice City for committing murder.
The kicker is that in GTA 5, players must rescue Bottino's daughter from gangsters who are trying to bury her alive as retribution for Sonny wronging them. We learn that Bottino "ran the East Coast between the 80's and 90's". Considering his date of birth, he was around 20 years old at the time. It would take some impressive skills and a very interesting life to run the entire criminal establishment of such a large area at such a young age.
The East Coast also includes Liberty City, base of the Gambatti family, and Vice City, location of Bottino's murder - and, more importantly, the location where the most believable and widely shared GTA 6 leak indicates the game will take place. That particular leak claims there will be limited missions in Liberty City alongside the main action in Vice City, and time-jumps in the story - all fitting this theory.
All of these dots were connected by a Reddit user named 'EMPIRES' and it makes a great deal of sense. For their next game, Rockstar might opt for a more serious mafia story documenting the rise and fall of Sammy Bottino, who left his mark on many other games without appearing even once.
It's possible that this setup all leads to a playable protagonist, whose absence so far is due to Rockstar wanting to make sure they aren't forced to switch designs and voice actors by the time we're actually given the reins.
On the other hand, it might just be wishful thinking and seeing connections where there are none - but as far as fan theories go, this is one of the best we've heard so far.