Wait- the mechanic is "cities working natural wonders receive bonuses". But:
1. The bonus must be for the city itself? Does a UA containing "units trained in cities working natural wonders gain +1 strength per each citizen present in the city" fit the mechanic?
2. Can that bonus be dependent on other things? Like: "X generates food for cities working natural wonders".
Anyway, my design:
Viceroyalty of New Spain
Antonio de Mendoza
UA: New World for the Old Man
Units in foreign lands gain +1

strength and grant +1

great general points from combat per each tile separating them from the coast. Cities working natural wonders provide

happiness equal to the wonder's highest yield in addition to receiving

culture and

golden age points equal to the yields of luxuries in their territory.
UI: Hacienda
Unlocked at chivalry, and may only be built on flat plains or desert tiles, never adjacent to each other. +1

culture, and +1

faith if worked by a conquered city. Generates +1

production,

gold or

food per each adjacent resources granting those yields. If adjacent to a natural wonder, provides double yields and increases its yields by +1(2 if it has only one yield).
UU: Conquistador
Replaces great general. May fight like a normal unit, with

strength equal to the strongest cavalry unit you may train. Though it only grants +10%

combat bonus(doubled in foreign lands) it may be consumed to assimilate conquered cities, granting a large sum of

golden age points and causing land units to ignore ZOC and terrain costs in foreign lands for 10 turns(on standard).
Lol I think I might actually play this design.
P.S. tell me what you think about the mission, I might replace it but I think it fits the design.
(Its supposed to make the player go liberty\ exploration)
(and for what its worth, I had a totally different mechanic in mind for that design)