The product strategy should give freedom of maneuver and at the same time define clearly the borders behind which stretching the brand performance is not a viable option.  Just pay attention to not define it too narrowly.

Ivory soap lost its market appeal because P&G did not leave enough room for maneuver saying Ivory soap had to:

• Be 100% pure (actually 99 44/100%)
• Have Ivopo (a proprietary perfume)
• Float

Game over. All you can do is keep communicating the brand promise, but time plays against you.

Innovate or die, wrote Jack Trout.