Madril

Gil Galad maybe as a summon, though I don't see him fit in that era. Cirdan, yes, Gildor, maybe too! 

Yeah but that's incorrect. They climbed the mountain but they did not enter it. They may have do it, but they were certainly enough to do so while they had to fight the Free Peoples. In BFME2 they were wrong, they got attacked by Easterlings not by Orcs (or at in a lesser extent). Read the appendices, it's said that  Dale and Erebor had to fight back against the Easterlings. 

Wow nice!  

It wouldn't be that good, for the simple reason you're melting every Second and Third Age battle with another. By the way, Orcs did not take Erebor, it was the Easterlings who tried to invade it but got repelled by Dwarves and Men of Dale. The Orcs of the Battle of Five Armies did not enter Erebor if I'm correct. 

I'd say Dol Guldur, I always love Orc factions and since Lorien is already here, Dol Guldur should be a good rival for the Galadhrim. In the other hand Isengard will have a strong opponent of the name of Imaldris, with advanced technologies and knowledge as well as very strong elite units.