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.
This is still being worked upon. I hope to have more details in the next couple months.
Please know the team is actively working on this feature.
This is very important. People who've known Outlook Tasks and iPhone reminders over task with little things in planner, don't get line items in Planner and don't know to follow up on small line items in Planner. Many of my dept manager level users feel feel like Planner is an island they don't under stand if their iPhone doesn't remind them.