Currently there is no plan to port turtlesim. Our engineering time is focused on implementing additional features and migrate packages which are requested most to be used for applications. If anybody would like to work on porting turtlesim
please feel free to create a pull request for it. I am happy to review it.
For the next ROS 2 release Crystal scheduled for Dec. 2018 we are working on porting gazebo_ros_pkgs
as well as some of the plugins. That will provide native integration with ROS 2. Until then you can use Gazebo with the ROS 1 integration together with the ros1_bridge.