I love the G19 size. That 1/2" difference is a huge difference for CCW. Atleast for me and quite a few others. It just makes a perfect all around size.
I don't see why they would not offer a mid size M&P. The M&P is proven and many contracts are starting. It's popular and not another Sigma POS thats not worth the investment for S&W. For Glock civvie sales the G19 is the #1 seller hands down. Even some LEO agencies have adopted the G19. Also Blackwater issues G19s as well as the Iraqi police and judges within their judicial system. It's popular for it's size, weight, and mag capacity.
With the success of the M&P I'm quite positive they will offer a mid sized. I just don't know when.
When they do offer one I want a .40SW. Hopefully the M&P will follow the route of Glock; conversions and a plethora of aftermarket parts will come out as well. Along with a wide variety of conversion barrels.