I finally played Conviction a bit ago. It... wasn't terrible, but I don't understand why they changed so much stuff.
I hope the next Splinter Cell basically takes Chaos Theory and adds the handful of good ideas from Conviction. Climbing stuff by pointing at it and pressing A was good, as opposed to jumping and hoping that Sam grabs on like in the older games. The cover system worked well, and even added to the stealth aspect. And keep the mark & execute, for people who want to play that way.
But give me back nonlethal ways to take out enemies. I should be able to play through the game without killing anyone, or even ghost through the game without even interacting with any enemies, except the ones who are necessary for the plot. And bring back the big, relatively open levels that let you accomplish your objectives in whatever order is convenient, as opposed to the completely linear Conviction levels.
I prefer the Chaos Theory style of play, but there's no reason both it and Conviction's play style couldn't both be viable options in the same game.
Oh oh, and don't make the enemies wander around saying, "FUCK SHIT FUCKING FUCKITY SHITCOCK FISHER!!!!!" That just felt like a child trying to sound like an adult.