Who are the Elders? In the first game talked about the fact that the control warp is the most primitive their technology (the murder of the commander of the sectoids). Also in late we have 4 avatar (one of them not completed). 4 God of chaos and 4 are chosen (leaving the occult part of what is happening can get a good Headcanon).

[Play_X2_XP_ASSN_T_Lost_Turn_Begins_C AkEvent]
SpokenText="I wouldn't have thought that an even lesser form of human could possibly exist."

Plot. The Imperium may not be the only one, and the Necrons may not have come from our space at all. All we know about them are legends. In Warhammer there are also 4 K'than (perhaps they are the gods of chaos). Why would the Elders help them? Perhaps they themselves are obsessed with Warp or psi network allows their ships to travel. This opens up a huge space for imagination. For example Eldar (not Elder) can be selected or changed once the elders of the people. They may develop the race no one plan aiming to create the perfect psionic, but hitherto they have not come out. It is possible to Necrontyr Elders themselves were once those who fell under the influence of K than the gods of Chaos and that is why they need a human body (sogeval Necron).

In the story of our world the Empire was destroyed by the Elders. At least that part of it which will be discussed in the narrative.

Another idea. The elders destroy all non-human species or take them away. There may be many hundreds of Lands like ours, and in many hundreds of Lands they have failed, but their time is coming to an end.

Spaceman is not at all an ally of XCOM. He also claims to dominate the planet (because it is in some measure a copy of Terra in another dimension). I think it's worth adding a faction of Space Marines, as well as the achievement of "Superman" - "Kill Space Marine with your bare hands." That is without a sword. How to do it - think to you.