What a fascinating idea. Looks pretty cool — if I ever get back into vanilla Minecraft I'll give it a shot (I've been 100% modpacks for a few years now). Any plans to support some of the more popular modpacks? I can imagine that the server costs for that could get brutal though.
These days I'm thinking of getting into minetest (http://www.minetest.net/), but the mod scene is pretty primitive there.