I don't think that it's a matter of "sandbox" vs "theme park" design. I think it's mostly due to the media games ate being presented on. We, as enthusiasts simply need "more better" input.
When I first started playing games it was on a black and white tv, my first RPG(to me) was
"Adventure" on Atari....but I " felt" like I was in game...my imagination was at work...
I didn't get that feeling back until I started playing MMOs not too long ago...(2005?)
First time I went into Azeroth on a 15" CRT monitor, I was like, literally blown away...
Fast forward to today....game designs aside, I don't get THAT feeling anymore..
I'm not burned out on the genre....just the way it's presented. I have(and believe most of us have) become spoiled or my senses have dulled to the game worlds.
So no matter what kind of game, be it sandbox or theme park doesn't make a difference to me, so long as I can get that old familiar "feeling" back...And I don't think that will happen until a new form of media is used for presentation..
Virtual reality any one? Maybe some day...