Aren't tile requirements and adjacency to mountains or rivers for wonders enough? I mean, why does a wonder have to be next to a certain district anyway? Does this add anything gameplaywise?
Losing the production and having higher building costs as the "new" district will be built later are enough drawbacks that building a district somewhere else won't be abused.
The only abuse I could think of is destructing a district and building another one ( e.g.: destructing a holy site for a campus ). But again this would add a new option. If the AI would place districts better, then there would be no need for relocating districts - I mean, the only reason to put the harbor where it is would be build two wonders left an right to the harbor - which should be no reason, as one could build one of those wonders in another city.
As you can change your cards, why not also your district placement?