Have support for ad units that always take a fixed place in the viewport.
See IAB mobile rising star - Adhesion for example: http://www.iab.com/guidelines/mobile-rising-stars-ad-units/
or
https://support.google.com/adxseller/answer/6068103?hl=en

The rational for doing these would be that when done properly (& with appropriate checks & choices) they have higher viewability while being minimally disruptive for user experience

Issues to discuss and agree would involve things like (not exhaustive)

• What should be the allowed size? What should be the behaviour on orientation changes?
• What should be the impact on click / tap?
• When should the ad show into view & how should it be closed / dismissed?
• Should these ad units only be allowed when other in-line ad units are not present on the page?
• Should it be allowed to refresh these ad units? If so how many times, how often?
  etc.