Innovation within a genre is hard enough, but innovating within your own franchise? That's a very tricky proposition. If you don't change enough, your title might seem irrelevant or even cynical; change too much and the core of the title – the sprirt – can be lost, along with your fans.
It's a delicate balance – and one that Irrational honcho Ken Levine is very familiar with. The developer's latest title is BioShock: Infinite – a release that appears to hold only a cursory similarity with its predecessors. However, as you'll read, the heart and spirit of the series is still there – and much more.