Simple Fort improvements:
1) retain the 50% defensive Bonus, add upkeep cost of 1 GPT (+1 when Flight is discovered)
2) forts constructed on resources INSIDE YOUR TERRITORY will connect them to your trade routes
3) forts connected to at least 1 tile of ocean/lake will be able to house 1 boat unit. Forts now have incredibly important strategic value
4) forts will be able to house 2 air/missile units. Also adds to strategic value.
5) forts should provide a 1 tile ranged attack equal to the DEFENSIVE strength of any unit stationed inside it. This will eliminated the ******** situation of having to leave your fort to destroy a weakened enemy unit outside it. It will also make forts useful against naval units.
6) all these should be true of Citadels as well, except of course Citadels are much more powerful.
7) constructing a fort should automatically construct a road on that tile as well. It's annoying having to build a fort them build a road on the same tile. Just a personal preference.
8) forts may be attacked by melee naval units