The verb usually agrees with the subject noun.
What she really wanted for her birthday was tickets to Rihanna’s concert.
All she ever wanted was diamonds.
However, if the verb is a long way from the subject and closer to a complement, the verb can agree with the complement.
What she really wanted her mother to buy her for her birthday was/ were tickets to Rianna’s concert.
All she ever wanted to get from this stormy relationship was/were diamonds.