Good news, this feature is partially done. :)
Planner tasks can now be copied to other plans. To copy a task, click on the “…” menu on the task. From there, you can select a different plan to copy the task to.
There is also this request which has a lot more votes. https://microsoftteams.uservoice.com/forums/555103-public/suggestions/16926463-need-full-capabilities-of-planner-in-teams
This request is still on the backlog.