Look guys, I don't have the game yet. Maybe I have a cleaner view of this than someone entrenched in the game deeply. Based on GTA IV and the directions taken in GTA V I say they are preventing you from heists, massive character overhauling like safehouse buying and anything you feel limited by in SP and allowing you to treat GTA Online as it's own experience vital to your game, a true 2nd world to your game.
GTA V SP gives you so much more than ever before, in the series, no matter what you feel limited by. They are bringing you FAR more customization and control of your decisions, life and character in GTA ONLINE. SP is going to always be a story mode, using built-in characters and NPC so you can't distort the mode too much from where they are going to place it. IE: Michael can't leave his family buy a house in Redondo Beach and say f*ck it, Im marrying a stripper GF from Bahama Mamas West. Trevor can't leave Blaine County and be the idiot neighbor in Rockford Hills, that doesn't match the SP mode. They are allowing you to that in GTA Online to the fullest extent.
That's why I also brought up GTA IV, because yes it's true you can't take Johnny Boy from the clubhouse reform him and move him into South Algonquin like it's cool. No Niko Bellic will never be a true millionaire living it up, driving custom vehicles running Liberty City but you get opportunity with Luis but there are also restrictions in his life as well. They are going to be a little strict and try place importance on their gameplay mode in SP. However, this time they are now giving you a better opportunity than you ever have had before, and you get to create your own ways in GTA Online. This is a new generation of grand theft auto, far more highly advanced with it's roots still in place but they are doing these things in a careful way while still giving you everything that you want.