I think the key between breaking new ground with a series and sticking with its roots is to release both as entries to the series during the course of a generation. I guess that one could say that a series needs a proggressive part (innovative and fresh) and a reactionary part (based on the series roots).
An example for me would be Mario platformers. In the 13/14 years (depending where you live) there were no 2d Mario platformers between SML2 and NSMB. When I was a gamer in the later half of those years, I wanted a new 2d Mario so badly, I didn't really care if Mario games stayed the same. Now that 2d Mario has been revived as such, as long as I get my generationally or bi-generationally fix of 2d Mario I'm happy for Nintendo for innovate Mario platformers in any way they wish.
I feel somewhat the same with traditional FF games, FF: 4HoL filled my need for an old school medieval fantasy styled Final Fantasy game.
