This looks pretty cool. I tried it once, but even with flood plains set to "normal" I was getting them on plains tiles, and rarely on deserts.
Some suggestions to think about:
* A setting which puts flood plains on all desert river tiles, but not anywhere else. I often go into the world builder and fix the floodplains to make this true on any game I play anyway.
* If it's not already in there as the default (my experiment ended when I ran into so many plains flood plains but I notice some of your screenshot maps seem to be a little less icy), an option to reduce the number of near-useless tiles in the polar regions. The game insists on trying to start players on the tundra, and that makes for either a weakened opponent or a too-hard game. Personally I would consider getting rid of land ice tiles completely just fine, but just reducing the proportions of both tundra and ice substantially would be nice.
* I don't mind having the land go near the edges of the map, and a few sea ice tiles for flavor, but I'd like to be able to guarantee that northern and southern passages are possible; this makes trying for circumnavigation much less of a gamble and more of a concerted effort. (Although the BtS clarification that it's your maps, not an actual circumnavigation, that matter makes this less of a problem.)
Both of these latter two are also part of making the cylindrical map make sense as an approximation of a sphere. The core approximation in a cylindrical map is that you presume that the north and south edges are where the world becomes impassible, and conveniently that this happens at a low enough latitude that you can ignore the lack of modeling how much shorter a parallel is at high latitudes than low ones. Having your glacier-covered area extend at all into the playable cylinder, or your tundra extend much into it, gives up both playable terrain and the quality of the approximation.
