I'll consider a big seller, a game that can sell 20mil+ without bundling.

GTA is still a big seller this gen so I think CoD will continue selling tens of millions next gen. As for the next big seller. Its hard to predict what the big seller will be because it will be a game that takes advantage of the new features the consoles will offer.

GTA became really big last generation because consoles finally became powerful enough to support a 3d sandbox "real life" game. It was basically the sims for people who enjoy action movies. 

Call of Duty became huge this gen because online multiplayer on consoles became viable for nearly everyone. You could argue online multiplayer was viable on the original xbox, and it clearly was considering how well halo sold, but the xbox install base was far too small to generate a 20mil+ seller, and playstation 2's online wasn't well done, with no centralized system.

The new big game next gen will be something that cannot be done well in current gen consoles, or it would have been a big game this generation. What that is, I don't really know. 

CoD does yearly releases, so it is pretty much guaranteed that a CoD (possibly Modern Warfare 4) is going to be a launch game for the PS4/720.

So yeah its still going to sell very well.

The one thing that all those franchises have in common (aside from no longer being as popular or prestigious as they were in their heyday) is the fact that at some point they started to decline not in popularity but in quality, culminiating with an installment (or two) that was so far removed from what made is predecessors great that it pretty much tarnished the entire reputation of the brand.  The last couple of Tony Hawks and Tomb Raiders were the biggest offenders here, and there's been a few mediocre NFS, FF, GH and Crash games over the past generation or so.  I'd say Tekken is the only series that doesn't belong on that list... the only reason it's not as popular these days is because fighting games in general aren't that popular, aside from the recent MK/SF revivals.

Love it or hate it, the fact of the matter is there has yet to be a bad installment in the main COD franchise that derails the series and sends it into a downward spiral of mediocrity.  Sure, there's been a couple of stinkers on handhelds, but each new installment in the main franchise has been bigger and more successful than its predecessor, culminating with BLOPS 2 which some would argue is the best COD yet.  And with the next generation of consoles going into full swing over the next year or two, this is only going to push the COD franchise even higher into the stratosphere.

Yeah MW3 had a chance to hurt the franchise but the good reception to BO2 pretty much has brought the series back to being as well liked as ever.

I am hearing far far less hate and push back against COD this year compared to MW3... and I think its because Personally and from what I heard from my friends BO2 is a solid game.

so COD will only decline if they have a really really bad installemet or have 2 bad installments in a row.