As an astro-imager, I want more pixels. My telescope plus various optical components produces an image circle that pretty much covers an APS-C sensor. So I'll be perfectly happy in a couple of years when APS-C hits 40 MP, as it surely will.
Based on pixel density, I wouldnt be so sure you'll see a 40MP APS-C camera. Heck, even the D800's sensor (made for high MP) only captures 15-16mp stills in APS-C mode.
If memory serves me, the current Canon APS-C cameras are about 40-45mp as a full-frame equivalent. Making a 40mp APS-C is probably like making a 100+MP full-frame. And while I'm not sure of the particulars of your astro-photography, realize that you will likely lose any low-light ability. If people think the 7D is bad at ISO1600, they'd hate your camera at ISO400. You'd be giving it the light gathering ability of your average P&S. It might not matter for you if you can infinitely decrease shutter speed, but for most APS-C users, it'd be a deal breaker. You'd get your resolution, but be sacrificing noise performance.http://www.dpreview.com/learn/?/Glossary/Camera_System/pixel_density_01.htm
edit: To take this further, the 24MP D3200 has an advantage in terms of resolution relative to the field of view. For the D800 to get the same shot, it'd produce a lot less resolution. That's because the D3200 has a pixel density that is 50% higher than the D800. When the ISO numbers come out, it wouldn't surprise me if the D3200 is worse than the Ti cameras in low-light ability.