Huitzil does have a point -- it's one of the difficulties in converting Touhou to a Civ game. Most of the factions in Gensokyo are pretty non-expansionist, but Civilization is a game all about expansion and competition, so it comes down to a question of trying to find a balance between aggressive and defensive.
I could certainly see Yuuka is being a "tall" sort of civ, which stays on the defense and works towards other victory conditions (probably a culture victory, although I suppose diplomatic could work?). On the other hand, there is a point in that fanon considers Yuuka to be one of the more violent characters of the setting, and it does make a certain sense that she might consider converting the entire world to a garden could potentially suit her.
I love the Polder idea, and replacing the Garden seems rather appropriate, although one would likely have to pick its bonuses according to what victory condition you wanted her to pursue. The issue with that, though, is that then you don't have a UU, which is pretty rare, I think.
I'm not quite sure what would make a suitable trait for her, myself... I'll have to mull that over a bit.
Also... has anyone considered a Makai civ yet? The thought occurred to me earlier today, but I don't really know enough about the PC-98 games to say for sure what they'd be like, although I'd guess Shinki would be the obvious leader. The events of Mystic Square make me wonder if it would have some kind of tourism effect, although that'd have to wait until BNW came out. I suppose one could view Makai cities as being the various portals/gateways to Makai, so it'd make sense as a "tall" civ, in that regard...
Come to think of it, I wonder if the trade route/tourism effect would be interesting to incorporate into the various shrine-related civs? The whole "visiting the shrine" concept... so the Hakurei Shrine might milk trade routes and tourism for more money, while Moriya could do it for faith...?
EDIT: Also, I think I'm going to knuckle down and study the "attrition" effects in some mods... it could be useful for things like Makai's miasma, or even the carnivorous plants in the Forest of Magic. (I'm sorely tempted to see if I can make forests in the Forest of Magic cause attrition to enemies...)