I think there are tons of franchises that haven't really justified sticking around for as long as they do, especially the ones that release really frequently.
CoD would probably have to sit at the top of the list; it's had something like 10 entries in the past 10 years, and the general consensus seems to be that it's well past its prime now. Other multi-platform ones that I think could reasonably be put to rest now are Battlefield, Assassin's Creed, Need for Speed, Metal Gear Solid. Also Elder Scrolls and Final Fantasy; it would be great to see Bethesda and Squenix pour more resources into making new games, even if they stayed within the RPG genre.
Nintendo-exclusive... I think the company's pretty good at keeping most of its franchises reasonably fresh, but I think they could get rid of New Super Mario Bros.. The other games within the Mario universe are generally a lot more creative, and it's not like axing NSMB would leave the console starved for platformers (they've still got the other Mario games, plus Donkey Kong, Yoshi, Kirby and even Rayman). Oh, and I agree that the Mario & Sonic Olympic Games titles could stop.
Sony-exclusive? Gotta be Killzone. I think they could reasonably move on from Uncharted and God of War, too.
Microsoft could stand to ditch some of their shooter franchises, such as Halo and Gears of War. They've got newer ones like Titanfall now, and I'd love to see them get some more core IPs that don't revolve primarily around gunning things down.
