I mean, if you want to bring Men into Beleriand within the first 2-3 episodes, that could work for purposes of pacing, but I'd want to be real careful about potential Butterfly Effect issues. Crunching the timeline had a bunch of repercussions this season (and last) which I'd rather not repeat.
I want to second and third this a hundred times. Mangling the timeline in Seasons 2 and 3 has caused endless problems, some major. We should stick to the published timeline as much as we can (granting that there are a few different post-LotR versions of some dates.) to avoid causing massive, unanticipated problems that could potentially follow us through many seasons to come. And I agree 100% with MithLuin that the Dagor Bragollach won't have the necessary emotional impact if it seems to happen too soon after the Dagor Aglareb and the defeat of Glaurung.
This is the timeline that I have compiled: (making use of the Grey Annals but also some later sources that incorporate some changes and additional details)
5 Fingon and Thorondor rescued Maedhros. Reunion and reconciliation of the Houses of the Ñoldor.
6 The Ñoldor set a watch upon Dor-Daedeloth, sent scouts to explore Beleriand and the northlands, and met many Grey-elves. Finrod sent Angrod as an emissary to Doriath.
7 Maedhros waived his claim to the High Kingship, and Fingolfin became High King of the Ñoldor.
? Turgon’s folk settled Nevrast, the House of Finarfin settled Tol Sirion and Dorthonion, and the House of Fëanor settled northeast Beleriand. Building of fortresses/palaces of Himring, Barad Eithel, Vinyamar, and Minas Tirith.
?
Fall of Mankind (Morgoth and/or Sauron involved) (this doesn’t necessarily have to be depicted, but the Edain do need to tell Finrod there is “a darkness behind us”)
20 Mereth Aderthad, the Feast of Reuniting of the Ñoldor and Grey-elves
50 Turgon and Finrod traveled to the Meres of Twilight, and Ulmo sent warning dreams to them.
52 With Thingol’s prompting, the Dwarves of Eryd Lindon stole Nulukkizdîn from the Petty-Dwarves, and gave it to Finrod. The Ñoldor and Dwarves began delving Nargothrond there, in secret.
53 Ulmo appeared to Turgon, who went forth and with Ulmo’s guidance discovered Tumladen
60 Morgoth returned to Angband. Dagor Aglareb and victory of the Ñoldor. The Siege of Angband began.
64 Turgon’s folk began building Gondolin in secret
65 The Ñoldor helped rebuild Brithombar and Eglarest. Finrod Felagund’s folk built Barad Nimras.
66 Galadriel revealed to Melian the death of Finwë and the theft of the Silmarils
67 Angrod revealed to Thingol the Kinslaying and the burning of the ships. Thingol banned Quenya, and the Ñoldor began to abandon it as a daily tongue.
102 Nargothrond was full-wrought
116 Gondolin was full-wrought. Ulmo spoke to Turgon, promising to protect Gondolin but warning him it would not stand forever, and bidding him leave arms and livery in Nevrast for Ulmo’s messenger yet to come. The folk of Turgon secretly moved from Nevrast to Gondolin.
150 First meeting of the Dwarves of Eryd Luin and the folk of Caranthir
155 Orcs attacked Hithlum and were defeated by the Ñoldor. Morgoth decided he needed bigger monsters, and starts working on creating Dragons.
260 Glaurung, the first Dragon, attacked Hithlum and was driven away by the Ñoldor
310 The Folk of Bëor entered Ossiriand and met Finrod Felagund, then migrated to Estolad.
311 Finrod Felagund returned to Nargothrond with Bëor as his vassal. Baran led the Folk of Bëor in Estolad.
c. 312
Battles in East Beleriand. Bëor fought alongside the sons of Finarfin and Fëanor. (this may not be necessary to depict)
314 The Folk of Marach entered East Beleriand, and dwelt in Estolad
316 Aredhel left Gondolin and came to Nan Elmoth. Eöl took her captive and married her.
c. 318 The Drúedain and Folk of Haleth entered East Beleriand, and dwelt in Thargelion. Soon after, Thingol banned Mortals from settling southern Beleriand.
320 Maeglin, son of Aredhel and Eöl, born in Nan Elmoth
330-380Many Folk of Bëor to migrated to Dorthonion and became vassals to the House of Finarfin. Many Folk of Marach migrated to the Nuath woods and Hithlum, and became vassals of Fingolfin.
355 Bëor died (age 93). This was a really big deal to the Elves.
369
Dispute among the Edain in Estolad, involving a demon doppleganger. Bereg and his followers left for Eriador. Amlach moved to Himring and became a vassal of Maedhros. (this might not be necessary to depict, but it shows the fact that the Edain are not universally enthusiastic about helping Elves, and those who did help the Elves did so willingly, they weren’t compelled by their leaders)
? Fingolfin made Hador son of Malach first Lord of Dor-Lómin, and Finrod made Boromir son of Boron the first lord of Ladros in Dorthonion. (probably between 364 and about 430)
375 Orcs attacked the Folk of Haleth, and were defeated by Caranthir. Haldad and Haldar slain. Haleth elected chieftain of the folk. Caranthir offered them his protection, but they declined to serve any Elf.
376
The Drúedain and Folk of Haleth moved to Estolad. (It may work out to skip straight to 391)
391 Haleth led her folk and the Drúedain across the Brithiach with much hardship. Finrod convinced Thingol to let them dwell in Brethil and the forest south of Teiglin.
400 Maeglin and Aredhel escaped to Gondolin. (Maeglin was 80, an adult.) Eöl met Curufin, followed Aredhel and Maeglin, murdered Aredhel, and was executed by Turgon.
409
Athrabeth Finrod ah Andreth (doesn’t need to be depicted)
455 Dagor Bragollach fought on 3+ fronts. Ard-Galen burned, Dorthonion and Himlad conquered. Barahir saved Finrod. Oath of Finrod to Barahir. Hithlum besieged. Sauron captured Minas Tirith. Orodreth, Celegorm, Curufin, and Celebrimbor and their surviving folk fled to Nargothrond.
Galdor Orchal became Lord of Dor-lómin, and Barahir became Lord of Ladros. (We need to mention this because Barahir’s brother was the previous lord, and it needs to be clear that guy had no son and Barahir is his heir.)
Turgon’s messengers began trying to build ships at the Isle of Balar, hoping to ask the Valar for pardon and aid.
456 Fingolfin fought Morgoth and died. Fingon became High King.
Dorthonion was turned into Taur-nu-Fuin. Emeldir led the Bëorian survivors to Brethil, and some went on to Dor-lómin. Barahir, Beren, and 11 other men stayed behind.
458 Orcs attacked Brethil but were defeated. Húrin and Huor were brought to Gondolin by Eagles, and later returned to Brethil.
(But it may be necessary to put this before the 456 events, so Season 5 ends with either the Gondolin stuff or Fingon’s crowning, and Season 6 starts with Taur-nu-Fuin, Emeldir's escape, and Barahir + Beren’s plight.)
Events at unknown times:
Sometime before Turgon starts building Gondolin in 64, Grey-elves emigrate into Hithlum and Nevrast in large numbers. At least some come from the Falathrim. (If we don't do this, Voronwe and Tuor's foster-father Annael won't exist at all. We _
need_ to do this.)
Curufin gets Angrist from a Dwarf.
Finrod foretells to Galadriel that he will swear an oath that will kill him, and leave no heirs. (I think it's important to Finrod’s characterization that he knew fulfilling his oath would kill him [and maybe knew when he swore the oath, too] but chose to do so anyway.)
Some time before the Athrabeth in 409, the failed romance of Andreth and Aegnor happened (in Andreth's youth).
Soon after the defeat of Glaurung, Telchar made the Dragon-Helm for Azaghâl. Later it was passed in turn to Maedhros, Fingon, Hador, and Hador’s heirs. But this history can be explained by a character in the story of Túrin.
Sometime during the Siege of Angband and/or before the Nirnaeth Arnoediad, we need to show that Morgoth has adopted a policy of capturing Elves, especially Noldor in battle. He enslaves them in Angband's smithies and mines, but also he puts some under the Spell of Bottomless Dread, and lets them "escape". The Elves learn the hard way that escapees from Angband are untrustworthy and treacherous. Eventually even those who genuinely escape are shut out and become desperate outlaws in the wilderness.
Morgoth also tries this with Mortals, and it works just as well, although they're too fragile to be good slaves.
He tries both slavery and the Spell on Dwarves, but it doesn't really work. They can be broken by torture, but not dominated.
Oh, and it would be a good idea to show everyday Elven and Mortal life in Beleriand during peace, and ordinary "common council" among the lords of the Noldor and the relations between the three Houses, and Galadriel and Celeborn's romance and marriage, and Galadriel learning things from Melian, and the cultural change over generations as the Edain learn things from the Noldor...