- Defender unit reqs for Attack_Bonus (currently the reqs use the attacking unit only)
- A Range value of City_Radius to specify "defender is within city borders" (rather than the current City range, which AIUI applies only to a city containing the defending unit)
As an alternative to (2), City_Adjacent (search any city adjacent to the defender's tile) could also work.
In either case, the range would need to only count cities belonging to the attacking player — the bonus shouldn't be given to enemies!
Stretch goal: I also want a Great Wonder, Dowding System, which acts as a Sector Control in every city. range=Player wouldn't quite do what I want, as it'd apply over the whole map rather than just "within borders of any friendly city", so I'd need yet another new Range value, Player_City_Radius, to mean "within City_Radius of any Player city" (and for symmetry one could also implement Alliance_City_Radius and World_City_Radius with the obvious semantics).
I realise this is all pretty complex and not very generic, so it's unlikely to happen. But hey, the board is called Wishlist