Thats brilliant, someone should make a Minecraft total conversion mod! ;)
lolno. it wouldn't be in any way better than minecraft other than being "free". Plus the entity limit for breakable/placeable blocks would severly limit it.
I'm thinking of making it into a mod actually, but no block interaction.. Maps made specifically for the mod and optimised properly (not individual cube blocks put together). But stuff like Minecraft HUD, mc-style door entities, mc weapons, etc.
Another guy was considering doing a collaboration map with me where only certain blocks hiding secrets and caves would be breakable.
that might work