I think the DQ series does a great job with mixing the old and the new... it offers several new features per game that mixes stuff up, but never changes the core tenets of the series ('vignette' style storytelling, turn based battle systems, fun but not overwhelming dungeons, lighthearted monster design). Most games in the series are relatively similar in quality, and usually the changes that are introduced don't sour people's opinions on the game that much.
Of course, most people see the DQ series and think its stagnant, but they are used to FF radically reinventing the wheel every single time, usually for the worse.