Whether initially built in or added shortly after, there's one mini-game Rockstar included in San Andreas but cut in GTA 5 that would be great for the next game.
Whether that's moderate in size compared to GTA 5 or to other similar games remains to be seen, but this will allow Rockstar to continually build on a presumably both single player and multiplayer game for years to come. It's worth noting that GTA 6 is reportedly taking an incremental development approach, meaning that when it launches, it will be moderately-sized. After all, not everything was actually possible in every game.
This plays no small role in all the hype and rumors for Grand Theft Auto 6, but while Rockstar intends to look forward, sometimes the best way to do so is looking back. Because of the way Rockstar crafts its games, it often feels like anything is possible in Grand Theft Auto, whether it was the older games or the newer ones.