What Du’a to Say after  Tashahhud?
Question

Are there any du’as which one can say in the final Tashahhud after al-Tahiyyat and al-Salah al-Ibrahimiyyah? If so, what are they? Some imams stay for a long time in the final Tashahhud and the people praying behind them do not know what they are saying.
Praise be to Allah.Du’a after Tashahhud
Abu Hurayrah said: the Messenger of Allah (peace and blessings of Allah be upon him) said: “When any one of you recites the Tashahhud [according to one report: when any one of you finishes the final Tashuhhud ], let him seek refuge with Allah from four things, let him say: ‘Allahumma inni ‘a'udhu bika min 'adhabi Jahannam wa min ‘adhab al-qabri wa min fitnat il-mahya wa’l-mamat wa min sharri fitnat il-masih id-dajjal (O Allah, I seek refuge with You from the punishment of Hell and the punishment of the grave, and from the trials of life and death and from the worst of the turmoil of the Dajjal (antichrist)).’” Sahih Muslim, 588.
It was also narrated that in addition to seeking refuge from these four, one should also seek refuge from sin and heavy debt. 
Al-Bukhari and Muslim narrated from ‘Aishah, the wife of the Prophet (peace and blessings of Allah be upon him), that the Messenger of Allah (peace and blessings of Allah be upon him) used to say during his prayer: “Allahumma inni a’udhu bika min ‘adhab al-qabri wa a’udhi bika min fitnat il-masih id-dajjal wa a’udhi bika min fitnat il-mahya wa fitnat il-mamat. Allahumma inni a'udhu bika min al-matham wa’l-maghram (O Allah, I seek refuge with You from the punishment of the grave and I seek refuge with You from the turmoil of the Dajjal and I seek refuge with You from the trials of life and the trials of death. O Allah, I seek refuge with You from sin and heavy debt).’” Someone said to him: “How often you seek refuge from heavy debt!” He said: “When a man gets into debt, he speaks and tells lies, and he makes a promise and breaks it.” (al-Bukhari, 833)
It was also narrated that the Prophet (peace and blessings of Allah be upon him) taught Abu Bakr al-Siddiq a du’a which he could recite during his Salah, as was narrated by al-Bukhari. Abu Bakr al-Siddiq (may Allah be pleased with him) said to the Messenger of Allah (peace and blessings of Allah be upon him), “Teach me a du’a that I can recite during my salah.” He said: “Say: ‘Allahumma inni zalamtu nafsi zulman kathiran wa la yaghfir ul-dhunoob illa anta, faghfir li maghfiratan min ‘indaka warhamni, innaka anta al-Ghafoor al-Rahim (O Allah, indeed I have wronged myself greatly, and there is none who forgives sins besides You. So grant me forgiveness from You and have mercy on me, for You are the All-Forgiving, Most Merciful).’” (790). 
This du’a may be recited in the final Tashahhud , after the prayers seeking refuge referred to above.
It was also narrated that after reciting the prayers seeking refuge, a person may recite any good du’a he wishes, and may ask Allah for anything he wishes of the good things of this world and the next. Abu Hurayrah (may Allah be pleased with him) said: the Messenger of Allah (peace and blessings of Allah be upon him) said: “When any one of you recites the Tashahhud , let him seek refuge with Allah from four things – from the punishment of Hell, the punishment of the grave, the trials of life and death, and the evils of the Dajjal, then let him pray for himself for whatever he wants.” (Narrated by al-Nasai, 1293).
And Allah knows best.