Dancing Hoskuld
Deity
Well, Civ V City States AFAIK can do these:
1) Give Gold for first meeting, more for first player.
Can be easily done
2) Give Own Resource to best friend.
Can be done by giving them 2 copies of one random luxory.
They will then keep one themselves and trade away the surplus.
The difference will be first come first served, since they won't cancel the deal and trade with another guy just because the attitude changes.
3) Missions to increase attitude
Not quite possible
4) Attitude degenerate overtime
Doable if you don't mind checks every turn.
5) Special benefits based on types, maritime, cultural and military.
This is actually the main benefit of them, and the main reason for keeping them alive rather than killing them off. Yet this is pretty tedious to code, and even if it is possible to code, tedious to teach the AI.
Thus, if 5) is not doable, then there is not much point in it, since AI will just kill them off like flies, since they only have one city and can't possibly put up a fight.
3 and 4 are possible but as you say 5 is the most important. Even in Civ 5 they get conquered but I think this only happens when one asks for another to be eliminated. Which is why it is not possible to just force peace between the city states and all AI.