So, Season 1 begins with Ainulindalë and ends with the War of the Powers. Casting for this season is complete.
Season 2 begins with the awakening of the Elves and ends with....well, somewhere around the fall of the Noldor. Since this season has not been discussed at all yet, that will be the next task for casting.
Domhnall Gleeson is Curumo, so he can't be Maedhros.
The plan is: 1. Season outline. 2. Episode planning, in which characters take form. 3. Nomination. 4. Polls. 5. The execs clear or veto the results of the polls.
Nomination won't happen until... Well it will take some months.