Plus Ultra
Conquistador
I wouldn't put that much into new system. In Civ terrain was always a critical part in replayability, I don't think the change will be that great. UAs are good if they multiply the number of strategies. For example, China could build wonders faster that's cool and surely sets their strategy direction to building more wonders, but it's completely independent from the land which dictates which wonders are better to build. That's an example of UI and land working together well.
From the different point of view, Firaxis plans to sell DLC with civilizations and this means interesting UAs need to be there.
Although Arioch is right when he claims we lack confirmation on UAs for Civ VI, Stealth_nsk very elocuently explains in this post why they are a key part of making each Civ different.