One major area on todays web site building is performance. Anything that could be achived out-of-the box on the performace area would be great. The old cache schema inherited from Drupal 7 is good but new elements could be integrated as some "static pages cache" a la Boost, or some "BigPipe" Facebook's approach that has been adopted on Drupal 8.
On the other hand providing automatic AMP (Accelerated Mobile Pages) pages would provide an innovative approach and wonderfull ranking on Google. A more adaptative aproach could be auto-detecting the client platform and having the capability to unset block or even complete layouts depending on the final platform.
This areas of competence may be complex and tricky, I whish the Backdrop CMS team could find simple approaches to offer a great performance without difficult configuration offering all users with high speed rankings.