Well, if the communities don't exist, then nobody would care if there was a wiki for it or not. We need to make sure we don't get so caught up in editing that we forget that nobody cares about what we're writing.
Communities for things we want wikis about should exist, and we should go to those sites and suggest a wiki, for which NIWA would offer support.
Otherwise, taking them from Wiki Farms is a better idea, so if there are wikis on subjects we are looking for on Wikia, we can help free those wikis.
But the best alternative is to do neither. Instead of choosing a subject and getting a wiki on it, we should find Independent Nintendo wikis floating around and support them.
While NIWA is big, it is quite possible that they've never heard of us. Maybe the users have been to Bulbapedia and never quite scrolled down to the bottom. Maybe they regularly use StrategyWiki but have never actually bothered to go to the main page for years. Maybe the wikis are just sitting out on the Internet with no contributions at all, but still have a reasonable amount of content. Maybe they don't understand the benefits of joining NIWA.
In my opinion, the best wikis to get now would be fanon ones (despite my dislike towards them). I know that Fantendo turned us down, but we should be able to find other fanon wikis out there to match our "canon" wikis.