We've discussed this for a while now. I think the plan is to re-purpose the "sidebar" code, so that a "sidebar" is just a block area, and there can be as many block areas as needed.
Ideally, a theme would register which block areas are available, as this could differ for each theme. And we'd have a few different types of blocks, like
BLOCK_ARTICLE, etc, depending on the scope that the block needed to render properly.