Kinda weird in the sense that some tiles are treated as other tiles, that's a really weird concept that doesn't really match up with the rest of the game. I'm not sure if it's even possible.
Also kinda boring in the sense that you still have a ua and a ui working together to form one ability. It's kinda hard to explain, but it feels like both of them is necessary to achieve a decent effect and I really don't like that. Same reason why I'm not really a fan of the Egyptian Burial tomb, if a unique can't stand on its own but require effects from another unique then it usually just end up looking underwhelming.
I see, they are hills and then more hills. I understand the 'boreness'. But at least they are more common. By 'Treat the mountain like hills', I wanted to sum up the 'can pass over mountains' and 'build roads in mountains' thing. Perhaps letting them yield +2 production, like unimproved hills do.
If we want to get something different from UA, then hills have to stay with the Terrace Farms, as they are naturally attached. The UA needs to be something that benefits from the Terrace Farm, but that could be used in any type of terrain. Extra food turns into more populated cities or more specialists. So, something around those things may work.
EDIT: Also, the other unexplored facet for Inca is their lengendary gold mines. Mines on resources giving extra gold?
EDIT2: How about
UA: Can pass over and build roads on mountains. Investing in buildings grants 1 turn of WLTKD in the city, 2 if Wonders.
UU: Slinger: Hit twice. +20%CS against units on lower terrain (keep on upgrade).
UI: Terrace Farm, build on hills, same movement cost as flat lands. Gets 1 gold per adjacent mountain and 1 production per adjacent improved resource, grants 1 food to adjacent farms and terrace farms.
Right now, only China has something for WLTKD, and it fits the peaceful Inca people.