The stealth Idea seems good. I know that on San Andreas, they only included a few features like that, which were hiding in the dark, and sneaking from behind and using your knife to preform a stealth kill. As well not to forgot, the burglary missions were almost the same, except that you couldn't kill anyone or you'd get caught. If they were going to put new Stealth features, they also should place sneaking features as well.
To me, I think leaning against a wall would be a good idea, so you don't get seen if the enemy is looking the way you're facing. Also, on certain missions were sneaking is included, you push the analog stick gently to sneak. That wouldn't be a bad idea if you're going to sneak behind someone. Also, not to forget, if you're in a forest environment, you can use the colour of your clothing to hide in bushes, trees or grass were ever you are.
Now thats pretty good.