A woman can become a Buddha in a future birth. Gautama the Buddha was once a woman when 1st thought becoming a Buddha was planted in her mind. [p8 Practising the Dhamma with a View to Nibbana, Ch1 Eminent Buddhist Women by Karma Lekshe Tsomo]
The Bodhisattva and Buddhas are always male so you have to be born in latter lives as a male to complete your Bodhisattva career and become a Buddha.
After the definite prediction of future Buddhahood you will always be male. Before that there is a chance Bodhisattva may change between male and female.
Though is modern times this might seem controversial Bahu Dhātuka SuttaBahu Dhātuka Sutta mentions certain things a woman cannot achieve:
He understands that it is impossible, there is no chance, that a woman would become a worthy fully self awakened one—this is not possible.
And he understands that it is possible, there is the chance, that a man would become a worthy fully self-awakened—this is possible.
He understands that it is impossible, there is no chance, that a woman would become a universal monarch—this is not possible.
And he understands that it is possible, there is the chance, that only a man would become a universal monarch —this is possible.
He understands that it is impossible, there is no chance, that a woman would attain the state of Sakra—this is not possible
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For more comparative discussion on this matter see: On Women’s InabilitiesOn Women’s Inabilities by Piya Tan and The Bahudhātuka-sutta and its Parallels On Women’s Inabilities by Ven. Anālayo
