I understand change isometric view can't be considered, so I read many post giving low-CPU suggestions : weather, presence and quality loots, doors, water (beyond 3D, a better texture, Until i saw map i thought water just was a glich from border map... ).
Maybe animate trees, or flying paper in town (i saw this in a GTA, but can't remember wich one, a very impressive effect of dirty suburbs due to flying paper) can give a living stamp to the world whitout changing world engine ?
there is possibilites, but i can only speculate on GPU/CPU usage :
- use "3D layer" other than for people : for loot or other "light objects"
- Use isometric : with animated sprites : anchor a sprite who become animated when displayed/used (doors, water, paper, trees). with multi aspect of sprite too : sprite of barricade lvl 1/2 are beautifull but give a VERY FEELING OF REPETITION, same for broken windows, but it's rare to find 3 or 4 windows broken close enough, All of this "high definition durty sprite (or texture)".