Notice how I'm saying stagnant. Not bad. A great game can be still be stagnant so long as doesn't reach the heights that it's predecessors set. Most franchises work in the way that the first entry or two is really impactful, but after that, they don't innovate nearly as much and repeat the same mechanics. Sometimes, there are major changes in direction that work like "Resident Evil 4". But otherwise, most go from "wow, this is amazing" to "hmm, this is good".
Gears of War judgment flopped.
Halo 4 was not nearly as impactful as Halo 1, 2 and 3.
Uncharted 3 failed to match up to Uncharted 2
Zelda has got in a repetitive rut in trying to copy Ocarina of Time
New Super Mario Bros has a total lack of direction
Yearly sports sequels: FIFA, Madden, NBA, etc.
Fighting game sequels (most of them)
Mass Effect 3 (where do I begin with this one?)
Let's be honest: GTA V was a ton of fun, but not nearly as revolutionary as older entries for their time
What sequels (esp in a series with more than 2 entries) haven't stagnated.