What would also be interesting for Silmarillion Total War, imho, would be some sort of "cooperation" system, something more advanced than the basic alliance system that one may find on RTW or M2TW.
On the first point, in the vanilla games alliance are litteraly worth nothing, one can enter an alliance with another faction and then stab him in the back on the next turn - I understand that it is historically accurate, since there was a lot of backstabbing and worthless alliances, but from my point of view keeping things this way would take away any political stability that Middle Earth realms could have.
I was quite impressed and relieved by the system that Third Age Total War had, the obligation to send a diplomat in order to break the alliance before entering war with an "allied" faction. I mean, in Middle Earth nations are not as versatile and unpredictable as in our world, there are definite alignments according to which kingdoms will behave and interact with others. So I really hope that Silmarillion includes a similar system which would avoid, say, Finarfin's House to go berserk on the Falathrim 1 turn after they declared an alliance.
On the second point, Middle Earth is also about great events of multiple side battles, and fateful alliances - I think about the Nirnaeth Arnoediath, or the Last Alliance, or the Pelennor Fields. Generally such agreements are a turning point in the history of ME, and sometimes I felt like mods such as LOTR TW for RTW, or TATW didn't exactly make me feel like that.
Don't get me wrong, the retranscription of Middle Earth was splendid, but I was really thrilled by the Crusades/Invasion system of TATW, and I always felt bad that there wasn't any other true cooperation between factions.
So my point is, that there should be scripts that the player could trigger, which would allow something resembling these Invasions - but not necessarily on the same terms or on the same scale. If for example, there was a specific alliance script between the Noldor realms, or between the Edain Houses, like some anti-invasion script, well that would represent quite accurately what I'm thinking about.
And of course, something like the Union of Maedhros would be totally comparable to an Invasion in TATW, so I really wonder what you're keeping up your sleeves about it
Ofc, I won't be angry if there isn't such an option on STW, for whatever reason. I was just saying out and loud my wishes for a great First Age mod