I'd like to see one of two things happen:
1) Merge the 1D and 1Ds line into a single pro body with a 45-plus point AF system, 10-plus frames per second, and best-in-class image quality and high ISO performance. Megapixels? I don't really care, as I'm sure Canon will squeeze as many megapixels as their next generation of sensors and processors allow. This would obviously be the flagship body aimed at working professionals who need both image quality and speed. As others have already asked, the big question is if a camera like this would sell for $5,000 like the 1DMKIV, or $7,000 like the 1DsMKIII? My money is on the latter.
2) Update the 5D with the a decent AF system similar to the 7D's with a bump to 7 frames per second. IMHO, this is the bare minimum Canon can get away with considering how badly the Nikon D700 trounces the 5D MKII in terms of AF and FPS. The 5DII is already a great camera, just painfully slow with a pathetic AF system, so Canon doesn't need a revolutionary update to this body. IMHO, the 5D line is best geared toward for static/landscape/portrait work, and Canon has already carved a niche in the market with it's balance of image quality and affordability, so there's no sense in messing with that formula. In order to remain competitive with Nikon, the price needs to stay at around $2,500.
This scenario would create a huge gap in the Canon lineup between the 1D ($7,000) and 5D ($2,500). Canon's stuff has been pretty lame the last few years, but who knows, maybe they still have some surprises up their sleeves. What about a full-fame 7D MKII, built in the same gripless body as the current 7D, but with the 1D's AF system, high ISO performance, and 8 FPS for $4,000? Now that would be a game changer. The current 1D MKIV and 7D are aimed at sports shooters and photojournalists, where tons of megapixels aren't as important anyways, so Canon could conceivably offer fewer megapixels, slightly fewer FPS, in a smaller and lighter body to help distinguish it from the flagship 1D/1Ds replacement.
Now for the high-end prosumer line, a camera similar to current 7D with a 1.6:1 sensor would more or less be the next 70D. There are more than enough Rebels to fill the void that would be created if the 60D were replaced by something more like the 7D.