WW certainly seems to want to be Regina, judging by her having moved into her castle and stolen her wardrobe. I think spurned love for Leo would be a bit boring as her motivation though. Far more interesting IMO if WW is the half sister who Regina never knew about, who had a rotten life, and who now wants compensation.
I also like the idea of WW and Eva having been sisters. Cora killed Eva and now WW is seeking revenge by going after Regina, just like Cora killed Daniel and Regina sought revenge by going after Snow. Though that wouldn’t really explain WW stealing Regina’s clothes. Could still explain her stealing the castle if she considered it her right, as the sister of Eva, who was the true owner of the castle.
Either or those options would be far more interesting than WW having missed out on being with Leo IMO.