Antonia is stronger than most feminized boys' names, reflecting the pioneer spirit of Willa Cather's classic novel My Antonia. Antonia has recently fallen off the Top 1000, which is an excellent reason for you to use it.
Antonia was borne by several prominent women in ancient Rome, including the mother of the Emperor Claudius.
A recent distinguished namesake is British historian/biographer Lady Antonia Fraser. It is also the full name of actress Nia Vardalos.
Antonella is a pretty feminization; Antoinette, not so much.