To keep the barrier for beginners as low as possible (reasonable) i would suggest to select the PHP version, that is shipped with all relevant lamp/xamp configurations a new-to-php coder would probably start with.
the default apt package for ubuntu 16.04 (lts) is still 7.0. I know in fall 2018 Ubuntu 18.04 (lts) will be available and it may install 7.1 or event 7.2 by default but there are other relevant distributions and packages.
Drupal (which i consider a major competitor) wont raise the dependency to 7.2 soon, this should be considered too.
Considering all that i would vote for 7.0 support for another release and raise the minimum requirement with the next major release.