Hidden crashed ship?
As reported by these two Reddit threads, it is a crashed Scrin ship, that, according to the r/cncrivals community leader Alex06, has appeared with the - at that point - most recent update to Rivals (which seems to be 1.2.0).
The same user mentions it is a throwback to Tiberian Sun, so I expect it should resemble this ship (but covered in snow):
source
In Tiberian Sun, the ship had to be secured and defended by the GDI (Mission 3 'Secure the Crash Site' / Mission 4 'Defend the Crash Site').