Anyway, a few ideas I'm throwing out here.
XCOM vs Aliens
An asymmetric Arms race between an XCOM expy organisation, and an Ethereal expy organisation. XCOM needs to protect and maintain the trust of their member states, while the Aliens need to gradually disable and take over the Earth. In typical XCOM style, battle will not be huge fights between grand armies, but rather small clinical strikes from the aliens than need to intercepted by the XCOM team.
Notable things
The basic structure of the game is a bit different from most traditional Arms Races, in that there's no linear map. The aliens have spaceships, and can land anywhere they want. A panic system replaces the territorial control of the arms race system, but it's more or less similar.
In order to make this work, the turn design is different. Both sides will not be taking actions at the same time, instead, they follow the following table.
Aliens XCOM
Phase : Revision Design
Phase 2 : Planning Attack Revision
Phase 3 : Design Planning Interception
Phase 4 : Battlephase
The Design, Revision and battle phase are similar to other games. The Planning phase is somewhat different. During the plan phase aliens choose which nations they want to attack with what purpose and equipment. During the Interception phase XCOM gets a list of what ships they've seen, and what stuff they can shoot down.
Another notable thing in the XCOM games is taking an enemies stuff and reverse engineering it for yourself. That will be introduced as well. As a reward for successfully completing missions, tokens can be granted that reduce the cost of various designs. Since resource improvements will be relatively rare, the token mechanic will be important to field expensive, powerful units.
The last thing is missions. In a way similar to the tactic mechanic that's used in Spire Race, missions are things that can be designed or revised and that alter the behaviour of your forces on the field, and which forces are best. A chryssalid is very good if you plan a terror mission, but if you want to infiltrate the governement, you may want to send a thin men or faceless instead.