Jonny49 said:
None. If people enjoy it, let them play it.
...even if it's something like FIFA.
I actually agree with this. If I were put in the position to kill any of these franchises, I would most likely not do it. Every one mentioned so far has their fans, and to each their own.
However, I still think that it's a very slim chance that Call of Duty or Halo can go anywhere good. Halo has a slightly better shot, since as people have pointed out here 343 Industries is comprised of many former Bungie employees, and there's many places it can still go from here.
As for some the others that people have brought up:
Any Sports Games: They have their fans, and they will keep buying the yearly installments as long as they keep all the stats and rosters current.
MGS: They tied up every loose end possible in #4, and there have been plenty of prequels and spinoffs already. Success of future installments unlikely, except amongst diehard fans.
Final Fantasy: As long as there's Gil, maybe Chocobos, and possibly Moogles, the series can pretty much do whatever it wants. Unfortunately, Square isn't quite utilizing the series' full potential, instead taking more-than-heavy cues from #7 in each installment.
Sonic: If they seriously restructure or downright ax Sonic Team, there's a shot. A slim one.
As for Mario and Zelda, these games have no established series continuity to worry about, so they can pretty much do whatever they want. Again, though, like the problem with Final Fantasy, Nintendo would rather emulate past successes with these series' rather than exercise their full potential. A lot of franchises like this have heaps of potential clamped down by the perceived need for fan service.