That is something I've certainly considered and it is technically possible. Rick and I have chatted about it. The issues are size an cost. There are several "true" mid range drivers that would lend themselves to this type of design. However, as Rick pointed out in another post, to stay in the nearfield below the 400-500 Hz. crossover to the bass drivers, would require another line of bass drivers rather than one or two stuck at the bottom.The next question would have to be, would the 3-way concept provide enough greater improvement in sound quality to justify the size and price. We probably won't know the answer to that until someone actually creates one. It'll have to have a room that will support such a large speaker (18"-20" wide) and a very understanding spouse/significant other.
Jim