What should a worshipper say during a long wait for the Imam to complete the prayer, after the Imam has given the salutation from the third rak'ah of Maghrib prayer, and the Imam is praying Isha?

The worshiper (ma'mum) waits during the final sitting for the tashahhud, which is a place for supplication (du'a). The supplication therein is not limited by little or much. So, once he has recited the tashahhud and the prayers upon the Prophet (peace be upon him), he should occupy himself with supplicating for himself and for the believers for forgiveness, asking Allah for Paradise, seeking refuge from the Fire, and other legitimate supplications, until the Imam returns and pronounces the salutation, whereupon the worshiper pronounces it with him.