My idea is on the left pannel i should have a link for completed tasks
when user click it should show completed tasks of that user in the
middle panel. When he clicks one task , a filled user task form will be
displayed on right panel.
monitoring and accessing the history is something I see better fit in Cockpit than in Tasklist. You can find a list of all plugin points in Tasklist here. You will notice that there is no plugin-point for the filter column, but it’s not something I would be opposed of. So if you want to, feel free to provide a pull request