Going back is unfortunately not possible. You can, however, change most things, except for how far you've gone on the continent map, where you've placed expansions and a few other things.
You haven't specified why you would want to delete your city but keep your buildings and items, and since you can't do both, you can:
- change your guild and entire friend list
- completely rearrange your city, especially now that the Reconstruction mode is available
- delete any GB you want or build new ones
- try a different playstyle (military city, diamond farm, recurring quest focused city, city built to look as closely to a real-life city as possible with decorations, cultural buildings etc.)
- delete every single building and start from scratch in the age you last were (not something I'd suggest unless you're in the Bronze or Iron Age)
My point is, it's most likely not too late for you to change your city or the way you're developing it.