1.A headdress with actual mystical functions: a relic from Dr.Strange's sanctum, the exact origin of the headdress could be that it was from Sokovia, or it belonged to withes before Wanda, but this will require it to appear in DS2 than WV.
2.Chthonic symbol: In whatever Alternative Reality WV is in, the headdress could be something like a "crown" symbolizing she is the heir to Chthon's power or something like that. Just a random thought, the headdress could both symbolizing Chthon's possession over Wanda and later it could be reversed as Wanda's possession/control over Chthon's power.