Absolutely, definitely achievements need to span the range of challenge levels. It's not as interesting to have a bunch of achievements that will likely be simultaneously earned anyway, though (overlap).
I feel it could be interesting, but would rather have actual feedback on the experience first.
In any case, the initial achievement list is done for now. We have 255 and any new ones will have to be added beyond Beta 6. But this thread can continue to serve its purpose as a repository of ideas.