Detailed discussion topic: http://www.gamesas.com/index.php?/topic/1139539-fallout-4/
Taking place roughly 30 years after the events of Fallout 3 the former Capital Wasteland now with access to clean water & agriculture, a thriving scientific community (Rivet City), a democratically elected government thanks to the efforts of The Capital Preservation Society (now acting as the supreme court) and the Temple of the Union (now a political party who advocate the policies of Abraham Lincoln) and joint defence forces in the form of the (now Elder Sarah Lyon's) Brotherhood of Steel and Rivet City joint security task forces (who now receive mandated taxes and subsidies in reparations for their protection and reconstruction efforts while maintaining their independence); the former Capital Wasteland while not a exactly paradise by any stretch of the imagination drew both immigrants and neighboring hostility from other regional powers as from out of the ashes of the D.C. ruins rose the New Columbia Commonwealth; a beacon of hope in the otherwise great darkness that had followed the Great War.
With limited access to electricity, communications, and basic provisions the new commonwealth faced many challenges however in order to confront these challenges and continue with the recovery and reconstruction effort in ernest Rivet City and Brotherhood joint forces were to be tasked with investigating reports of an old pre-war Nuclear power station located somewhere beyond the ruins of the Big Apple Wasteland as it is believed that this facility could once again power the (entire) region. Both groups were to be well compensated with the full the support of the now fledgling local economy. The investigation of course was to be lead by Rivet City and supported by the Brotherhood of Steel who were to be permitted to maintain exclusive rights over any uncovered pre-war technologies discovered in the ruins of the former Empire state as part of the terms of the agreement.
However Brotherhood Outcasts and Enclave Remnants who had been pushed out of the former Capital Wasteland 30 years ago by Elder Lyon's Brotherhood had also traveled north to investigate the ruins and went on to form an uneasy alliance after first reports of sightings of eBoS soldiers. After being driven from their lands before they were determined to hold the Big Apple Wasteland (together if necessary) so as to retain control of the treasured pre-war technologies buried beneath the ruins of New York for themselves and preserve their orders and heritage as the only non-mutated humans left in America. Their pact to undermine and eventually destroy the eBoS amounted to seizing control of the Nuclear facility and launching a nuclear assault on the Citadel and by extension the New Columbia Commonwealth where ghouls and mutants lived side by side with 'humans' which was as they saw it an abomination.
Along the lines of Fallout: New Vegas it would be cool if the player got to take sides between the factions (Enclave/Outcasts and eBoS/Rivet City and the locals of the Big Apple Wasteland) and eventually decide the fate of both the Big Apple and Capital Wastelands.