This exactly.

The rigid separation of zones drove me nuts when I last played SimCity 4. Here in Vienna it's completely normal to have flats, shops and offices in every building. As far as I know this is even more extreme in Asian cities.

What I would love to see would be a triangle slider for zoning where you could for instance say this block should be 60% residential , 25% industrial and 15% commercial. Maybe some industries or commercial venues will require higher percentages to settle but this could also be an interesting tool.