There's a couple of issues concerning resource tiles that I've stumbled upon.

• First of all, I think, they should be completely destroyable — perhaps, that process should also take some time.
• Is there really a point in showing player resource tile upgrades they cold never build (while there is no enabling tech in the tech tree at all)?
• Perhaps there should be more than one way to upgrade a resource tile, with different bonuses, which could be either empire- or city-wise? Of course if there could be more than one, their number should be limited, if not to a single one.
• A questionable idea, but maybe there should be a way to improve resource tile's upgrade right where it is, rather than or in addition to city buildings?