Shape Up was my first thought too. I just left a team where I introduced cycles of six weeks of feature development and two weeks of bug fixing, tech debt, and anything else the developers decided to tackle.

It depends on the stage and size of your team and company of course, but for us the result was more predictable delivery and happier, more-engaged developers.

For anyone curious to learn more: https://basecamp.com/shapeup/2.2-chapter-08#cool-down