Huh. I think from all of my games, GTA V/GTA: Online has the most of stuff to do. I have games like GTA IV and Skyrim (if talking about current gen games, that is. I have other open world games in PS2-era games)...
I think open-world games need you to have some sort of an imagination to keep on playing without getting bored. If you don't do anything else than farm Jobs in Online, you can bet it'll get boring. If you don't try anything different in GTA V, it'll get boring too. I am not sure about games like Mass Effect, that how open-world(ish) are they.
But anyway... you should try to do some special things in the games if they are open-world (why not linear, too). Like in GTA V/Online get a truck and get between a tunnel and a train. Or get a Rhino and do the same.
There are lots of things to do, it just needs you to realize the amount of stuff to do. In Online it's more fun because you have friends/other people to do those things with.
Plus... don't buy PS4 right after the release, mate. It isn't worth it; there are hardly any games coming at the launch. If Watch Dogs wasn't delayed then I'd buy a PS4 at launch too, lol.