Really, just think of the possibilities. The campaign mode for The Silmarillion could have three factions: the Elf/Man/Dwarf Alliance, Morgoth's Horde and the Valar Host, played in this order, like in most story-oriented multi-faction RTS campaigns.
Bear with me:
The game starts at the beginning of the First Age, with the first mission serving as a tutorial of sorts, in which you control the Noldor who left Valinor along with Feanor. The "tech tree" gets developed as the player progresses through the campaign, just like in any RTS, but trying to follow the novel's plot: you start with Elven warriors, as they get a foothold on Middle-Earth, then the Dwarves and Men join up as you reach the later missions, up until the Siege of Angband.
This setup could be quite interesting, using the Battles of Beleriand as a background for the game's plot. The first campaign ends with the Long Peace, and the second one lets you control Morgoth's forces, and just like with the good guys, your tech tree follows the novel's plot, with a weakened Morgoth having only orcs and lesser evil creatures, like wolves or wargs, and with you eventually gaining the ability to use corrupted Men and Dragons as Morgoth sends Glaurung with its full strength.
These two campaigns are the main plot, and then a shorter (and more difficult) third campaign wraps up the story, with the Valar Host coming to save the day after Morgoth overpowers the good guys. Think of this as a C&C 3 kind of setup, with the Valar campaign being like the Scrin in that game. I really think that could work, and it would be very entertaining. Maybe even more than the novel itself, which is an amazing, but quite difficult read.
Also, think of the playable heroes. Screw Aragorn, Legolas and the Witch-king. What I really want is to kick some serious butt as Feanor, then make Elf BBQ as Glaurung.
I have multiple, massive nerdgasms from just thinking about this stuff.
