Does events allow you to switch between the religions? Mainly for Saruman, changing him from "Light" to "Dark".
If we would calculate the scenarios EU-wise (400 years for GC) the game would start in 2621 T.A. I can remember the following events from that period:
2746: Amrothos, 15th Prince of Dol Amroth, falls while defending Dol-en-Ernil against the Corsairs of Umbar
2758 - Dunlendings, under Wulf, invade Rohan, supported by the Corsairs of Umbar
Long Winter of 2758-2759 - The Dunlendings lay siege to Hornburg
2799 - The Battle of Azanulbizar is fought on Moria's East Gate, in which the Dwarves defeat Sauron's minions but are defeated by the Balrog Durin's Bane
2885 - Harondor is once again claimed by the Haradrim, supported by the Corsairs of Umbar. Fastred and Folcred, the twin sons of king Folcwine, were killed during the Battle of Crossings of Poros
Fell Winter of 2911-2912 - wolves invade the Shire. Tharbad is ruined in the following floods.
July, 2941 - Bilbo Baggins obtains the One Ring; the White Council drives Sauron out of Dol Guldur
2941 - Esgaroth is attacked by the dragon Smaug, who is consequentially killed by Bard the Bowman; Battle of Five Armies
2951 - Estel, later known as Aragorn, comes of age and is told about his heritage; the Corsairs of Umbar officially ally themselves with Mordor and destroy great monument commemorating Ar-Pharazôn's victory over Sauron
2980 - Arwen pledges her hand in marriage to Aragorn; Frodo Baggins loses both of his parents in a boating accident; Aragorn, in the service of the Steward of Gondor Ecthelion II leads a taskforce south and kills the Captain of the Haven, ruler of Umbar; Sam Gamgee born4; Théoden, son of Thengel, becomes 17th king of Rohan after the death of his father. Théoden is the last king of the second line.
2989 - Frodo Baggins comes under the guardianship of Bilbo Baggins; a company of Dwarves, led by Balin, try to recolonize Moria
3001 - Bilbo Baggins turns 111 and leaves the Shire
3018 - The Ringwraiths are given the task of retrieving the One Ring; Gandalf returns to the Shire, telling Frodo Baggins he must take the ring away
September 30, 3018 - Aragorn meets Frodo Baggins in the Inn of the Prancing Pony in Bree
October 25, 3018 - Council of Elrond at Rivendell
December 25, 3018 - The Fellowship of the Ring sets out in the evening
January 15, 3019 - The Fellowship parts after Gandalf falls into Khazad-dûm while fighting a Balrog
February 3019 - Death of Boromir; Saruman launches a full-scale invasion on Rohan at Helm's Deep
March 3019 - Théoden King of Rohan is killed at the Battle of the Pelennor Fields, Éomer becomes his successor as the 18th king of Rohan. Éowyn and Merry kill the Witch King
March 25, 3019 - death of Gollum (Sméagol), about 589 years old; the destruction of the One Ring and final defeat of the Dark Lord Sauron.
May 3019 - Aragorn is crowned King Elessar of the Reunited Kingdom of Arnor and Gondor; Gandalf helps Aragorn to find the sapling of the new White Tree
(Date unspecified) - Faramir marries Éowyn
Mid-year's Day (between June and July) 3019 - Aragorn marries Arwen Undómiel
July 3019 - King Théoden is laid to rest beside other Kings of Rohan in Edoras
November 19, 3019 - Battle of Bywater; death of Saruman and Wormtongue
Spring 3020 - Samwise Gamgee marries Rosie Cotton and together they move to Bag End
3021 - Éomer marries Imrahil's daughter, Lothíriel of Dol Amroth
September 22, 3021 - Bilbo marks his 131st birthday, surpassing the Old Took
September 29, 3021 - Elrond, Galadriel, Gandalf, Frodo and Bilbo depart from the Grey Havens