Magna Carta
I suggest basing the quest upon one of the lesser charters which preceded both the more famous document, and the most dramatic of active hostilities between John and his Barons, rather than Magna Carta and the treaties of Runnymede.
There is a lot of potential in the politics of the surrounding events, though making it interesting would come down to your writers. I am unsure of M&B:W's ability to deal with multi-stage and branching quests so rather than suggesting specific execution I shall list some key elements worthy of consideration which I believe could be made into quests.
Key events possibly worth investigating:
John did his best to garner support from the pope, going so far as taking the cross (pledging to go on armed pilgrimage).
John confirmed London's city charter. Which gave a lot of power to the senior merchants in the city.
John demanded his barons take a new oath of allegiance.
John repeatedly tried to engage in negotiations and issued several charters but generally evaded and called for the arbitration of the Pope on difficult matters.
John gathered a large number of mercenaries.
John individually undermined the barons and their unity. It is worth noting John was well known to give extravagant gifts to his favourites and was accused of bribery more than once.
Other events worth consideration as quests:
The Braose conflict (1206-1211): Maud de St. Valery, wife of William, was jailed by the king for spreading rumours. William formerly a kings favourite and likely involved with the disappearance of Arthur has his english lands seized (supposedly for not paying taxes) and welsh lands forcibly taken by force of arms. William fled to Ireland and soon died. This treatment of a respected and powerful lord likely contributed to the tension with the barons. Given the nature of the game Perhaps William asks the player to talk/bribe John into reducing hostilities and releasing his wife.
Revenge for the 'Massacre of Abergavenny': In 1175 several Welsh nobles were invited to a Christmas feast at Abergavenny Castle. They were slaughtered. While some sources blame William's castellan for the plan, William Braose later hunted down and killed one of the Welshmen’s last heir, a 7 year old boy. In 1182 Hywel ap Iorweth sacked Abergavenny in retaliation. The castle was burned and was rebuilt from 1190-1215 during which time it frequently changed hands. Perhaps a quest given by a Welsh lord to capture the city, or to hunt down and kill the castellan might be in order.