After digging a bit deeper, it looks like ETC is really the only useful compression for Android deployments (since for all others you might end up with SW decompression, which makes no sense), but it works only on non-alpha atlasses (ETC2 is much better, but only available with GL3 and probably only on very few HW today)
So a great value prop for TK2D would be to support an RGB/Alpha split under the hood of an atlas, plus a shader that recombines them later on. Such atlasses could both be ETC compressed.
Not sure how much work that would be, but Android devs would love you for that!