Innovation is all about creativity. Let your creativity loose and you will see innovation coming your way. The problem with creativity is that you cannot encourage too much of it all the time.
Now your statement on requirements is true. I urge my team to look at the requirements and be creative within them to create innovation.
An example: If to build ABC you need 160 man-hours, to get 40% benefit. And with creative thinking you can build ABC with a twist of creativity in 180 hours, to obtain 50% benefit.
I think as a portfolio manager you have to focus on pushing the right values to your program and project managers.