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E17 would be fine, but it isn't ready yet, so you may come back to it later. Still, I'm not quite sure they offer some substantial degree of consistency anyway, so you can target a generic idea of lightweight UI for E17, FXCE and WMs with no DEs to establish consistent look and feel.
Yeah, that is more of a "it would be cool to do sometime in the future if I ever get the time". The initial focus will be on Mac, Gnome and KDE in that order. Than perhaps move to other DEs but probably not before I also have made an iOS, Android and Metro interface.
Interestingly Android will present somewhat similar issues as Linux does. There are several "environments" on Android like TouchWiz and Sense along with the "native" interface. If I want my app to blend in there there may have to be some considerations to those differences as well, even if it may be only "look" and not really any "feel" differences, allowing me to create a single interface and just skinning it basically.