The Coregrafx is just a redesign; not a separate console with its own games. Think Genesis 2.
They should have made Bonk's Adventure the pack-in game the second it was released. Also, AV out compatibility out of the box would have been a good idea, not to mention a second controller port for the US redesign.
NEC just dropped the ball from the beginning of the US launch and never picked it back up. So many great 3rd party titles were never available in the US. Games like Castlevania and Street Fighter 2 should have been on the shelves at Toys R Us, but by then it was already too late. They had a chance to be more like Sega with the Genesis, but never had the marketing or distribution they needed.
I agree with homerhomer about it being the mysterious system growing up. Fortunately I grew up in a house with brothers all reading gaming mags and we thrived on this stuff. You could rent a TurboGrafx at our Hollywood video (if only they had some post-launch titles to try) and we eventually got our own. Soon we bought used Lynx, Jaguar, CD-I, 3DO, Neo Geo on the cheap. TurboGrafx is probably what got us into importing.
@turboxray, I think NEC was just willing to accept any 3rd party games they could get, and they didn't want to push developers to make the games the most advanced. I've been playing Ninja Warriors, and it's a really by-the-numbers downgraded port. They certainly could have done better, but you had to take what you can get with early games on the system. NEC certainly did a much better job attracting developers than Sega did with the Mark III.
I like your idea of including the Super Grafx in the CD hardware, but it should've been all CD hardware, not just the Duo. Perhaps that's just too early to expect that, but it would've been a simple upgrade, much like the extra hardware included in the Mega CD, but much cheaper and easier.