Lily would be the obvious choice, but there are plenty of other options! Assuming that Primrose is out, given that you've already got a Rose, you might also consider Holly, Violet, Ivy, Angelica, Amaryllis, Clematis, Aster, Tansy, Dahlia, Zinnia, Camellia, Primula, Alyssum, Indigo, Calla, Marigold, Petunia, Lilac, Veronica, Lavender, Posy, Pansy or Viola. In Wales, Ffion (fee-on; it means 'foxglove') seems quite popular. I'm not quite sure that Myrtle is ready for a comeback just yet!
If you're willing to stray into the general nature theme, I like Laurel! Sequoia, Hazel, Cassia, Carissa, Clementine, Fern, Juniper or Willow could be options.
If you have a boy, Linden and Heath are nice.