Yeah it unlocks more activities as you level. Even the missions take on a different dynamic with 4 players as you can almost plan them heist style and have people take different roles. Especially once you done them once and know what to expect. Proper difficult heists where people have real roles would be pretty awesome.
Maybe they unlock as you get further in? I mean, the heists were a big part of the hype for GTAO, and there are playlists supported; it seems like they could do a limited-path or linear playlist with each stage of the heist.
I'm in the story with Trevor now, and... well, he has a different style of planning heists than Michael, fer shure.
One of the things which I'm surprised to enjoy so much about the game is the three characters. It is quite fun to notice the different way I'm playing when switching to each character.
When I play Trevor, I frequently find myself driving more aggressively, not giving a crap about consequences, and generally letting my id run wild.
When I play Michael, I worry more about losing whatever I've got.
When I play as Franklin, I get nervous when I'm in a nice neighborhood.
Also, not sure what happened, but each of my characters appears to have lost ALL their weapons. This was either after the heist, or after the Title Update. I also had read an exploit to visit Ammu-Nation and upgrade weapons during a mission, then fail the mission, leaving characters with the upgrades AND restoring the money they'd spent on them.
I'd not planned to exploit that, but I did find that I had started a mission with half-health and NO ARMOR, so I stopped by an Ammu-Nation, and IT WAS CLOSED. I'm seriously hoping that this is a one-mission thing. In GTA3 and such, there were so many times a visit to Ammu-Nation saved my butt and let me finish the mission.