So two semi-related ideas:
- Individual cities wouldn't be limited to one district of each type. In fact, building multiple districts of a certain type within the same city could even provide bonuses (adjacency or otherwise) – for example, two adjacent industrial districts could form an 'industrial mega-district' that provides a production bonus to every building within it, or they could just grant a major adjacency bonus to one another (but otherwise function as independent districts). Specialising too much could significantly impact cities' self-sufficiency if trade routes are cut off.
- Traders could enable more dynamic resource exchanges. For example, an internal trade route could siphon a customisable amount of food, production, etc. from one city to another, or even other resources like population (resettling temporarily or permanently). These trade routes could also be of flexible duration (indefinite, player can modify or cancel at any time). External trade routes could gain similar functionality, but maybe only with prior diplomatic approval. This would enable cities to establish themselves or function more flexibility, and would make trade routes more strategic targets (especially if cities within a civ overspecialise as above and become over-reliant on them).
- Players should be able to remove, convert and/or relocate districts. I know there's a mod for Civ VI that addresses this, but it should also be a default feature in Civ VII.
- If the multiple districts idea makes certain districts too OP/spammable, perhaps some districts could reduce tile yields – for example an industrial district might reduce food and faith yields, a religious district might reduce production and science yields, etc. There are also systems in the game now that boost the cost of a district based on the number of pre-existing districts, so that could be a simpler option (the more campuses you build, the more expensive they get civ-wide... or just per city).
- Districts should get more bonuses from the properties of the hex they're built on, not just bonuses from adjacent hexes. For example, a major production bonus for industrial districts built on strategic resources, a major gold bonus for commercial districts on luxury resources or road tiles, military districts could double a hex's defence modifier or gain bonus hitpoints from it, etc. This is already the case with preserves for example, which grant a housing bonus dependent on the appeal of the hex they're built on.