It's all dependent on what happens next gen (motion control? online still big?) and which franchise innovates and offers something new to the fans whilst gaining interest from others.
Looking at the examples you gave:
FFVI to VII: 3D graphics, FMVsequences, cinematic appeal to garner wider and mainstream interest
GTA 1/2 to GTA3: Took the basic gameplay and put it into a fully realised 3D environment. Driver 2 may have gotten there first, but GTA did it well and the previous notoriety and gameplay of the series helped boost it's sales.
CoD to Modern Warfare: The names say it all. Call of Duty was already gaining popularity as a good online game and top launch title for 360, but people were getting tired of the WW2 setting. Upgrading to the modern setting and weapons, but keeping the core and fun gameplay intact is why it has become so popular. Now, anything with Call of Duty in the name will sell even if it's a big steaming turd.
For next gen? It's very difficult to say. Possibly Crysis, but I'm not sure how well Crytek can innovate, and FPS' may not be so popular next gen. I hope Deus Ex does it, but I don't see it appealing to such a broad audience. Assassins Creed is already pretty big, so I'm not sure if it'll be able to top this gen. Uncharted is possible, but how can ND top Uncharted 2? Maybe Dragon Age depending on how Dragon Age 2 turns out. If Bethesda manage to release a bug free and polished version of Fallout or an Elder Scrolls and incorporate some community/multiplayer elements, I could see them becomming fairly big.
