People make long term investments about land as much as they do about cities - with the understanding that it is, as cities are, permanent. RoI that likely already takes years for that land to actually pay for itself. Scrapping permanency of land will make it meaningless, not to mention make any investment into it retroactively worthless. Blocking the use of it, or severely nerfing land/food output for XYZ time if the city were to be nuked would be a better option.
Increased improvement damage during wars ought to do the trick, if you're aiming for more destruction and making permanent war and resource farming templates unviable.
As for actual suggestions, I endorse Adrienne's QoL list. Many of those little things would make the game far more enjoyable and should be easy to do.
I'm not sure how I feel with moving update times, it further favours the aggressors, imo. Perhaps splitting the current update/buy into two and making units more durable (meaning you don't get insta wiped while you're sleeping) might be a better option.
One small thing I'd add, while trivial, would probably make peacetime more lively. Introduce Alliance Embargoes, as people have repeatedly asked for it. I know every time Sheepy struck it down as encroaching on individual nation's choices and sovereignty (lol), but perhaps those concerns could be alleviated by simply making nations able to "opt out" of any alliance embargoes on their Account page, like this was done: