I have really fond memories of playing the 600/800 series of Atari computer systems. Although we didn't ever buy a new atari computer system, we did find a couple complete setups at yard sales in that era, two of which had hundreds of (mainly copied/cracked/pirated) games, probably close to the entire library.
One of the games I probably played more than anything was Ghostbusters, but the Atari had a lot of good games for the time. It featured games on cartridges, floppy disk drives, and on cassette drives, and many were cross-compatible within the 400/600/800 series, possibly more.
Unfortunately, because of the huge series of atari computer systems, they would eventually have to make one (or more) that wouldn't be compatible. The Atari ST was really the system that split up the library more than any other computer system, which is where the commodore 64 really came in and beat the Atari. I would imagine that the apple II would have come close to the library, but the only system that could have beaten the C64 through the non-gui period of gaming would have been the PC-DOS era.
A very very large percentage of SSI/AD&D games were on the c64. Other computer systems (besides DOS) either had very little or like I said about the Atari, they were split between multiple systems. Pool of radiance, curse of azure bonds, death knights of krynn, dark queen of krynn, hillsfar, heroes of the lance, etc.. The C64 had games that spanned every era between the atari 2600 and NES.
If the C64 would have been modernized without a huge cost upgrade, then I believe it would have gotten another 10 years of life. The C128 came out in a few forms, one of which had an external keyboard and a desktop-like design, which I believed was the only good thing about it. The extra processing power really wasn't used for anything, and the extra costs make it obsolete about as fast as it hit the market. It was said at one time that the C64 was so successful that the company execs didn't care if the Amiga was successful or not. They had already gotten rich beyond belief.
What upgrades would have made the C64 more modernized? In terms of keeping compatibility 100% with the original c64, I don't think anything should have compromised it. Faster video/page refreshing would have been nice, via an upgraded video processor, with more hookup options: s-video, vga/svga. Obviously, the C64 was mono, and that could have been upgraded to stereo. Faster modems for BBSes and on-line services would have been nice, and compatibility with IBM/PC mice, keyboards, serial and parallel devices, and monitors.
The commodore 64 era also barely saw hard drives and 3.5" floppy devices, much less cd-roms. Having options for desktops or towers to accommodate this technology would have been great. The only computer system at the time that thought ahead was the ti/99-4a, with it's peripheral expansion box. Of course, that wasn't for cd-roms but was for floppy drives and all of the expansion cards.
At the time, the C64 was the system you'd get if you wanted it all. In this day and age, there is no equivalent to the C64.
105 HD-DVDS, 82 BLU-RAYS, 118 3-D BLU-RAYS