It's always hard to predict what will seem "unique" to any particular person.  It all depends on what you've seen and what you're envisioning. However, the situation you've mentioned, because of the fairly neutral colors involved, would be open to a number of different color and flower
options.

I'll mention a few types of color schemes that come to mind to me right away, but I suggest you go to your local library and look through some floral arranging books (with swatches of the maids' gowns in hand if possible) and just see what catches your eye.

~ You could go all-white (or all-off-whites, depending on your gown). This should still stand out with the maids' gowns, but it would be very
simple and "clean".  The champagne and gold color is very simple and classy, too, so it would enhance that.  You could use simple hand-tied calla lilies with gold ribbon, perhaps, or a more traditional bouquet with a mix of white flowers like lilies, freesias, roses, stock or
stephanotis.

~ You could keep it subtle but add in some color interest with varying shades of peach and apricot, perhaps even including a few oranges.  This could look especially nice if your wedding gown is ivory instead of white.  In May, you might find interesting shades in this color family in gerber daisies, alstromeria, lilies, roses, yarrow, glads, and freesias. I think yellow probably wouldn't look as nice, although some darker golden yellows might.

~ You might choose to go with a greater contrast in a complimentary color family.  Purples, lavenders or even blues would stand out from the champagne gowns.  Hydrangeas, larkspur and delphinium, scabiosa, statice, lavender and lisianthus all come in pretty shades about that
time of year.

~ For a really bold contrast, I would go with red-- probably a bright, slightly orangish red.  I can picture really bold hand-tied bouquets with a single flower type and color, such as crimson roses, gerber daisies or parrot tulips (check with your florist for tulip availability in your area in May). Again, with a wide sheer gold ribbon, it would be very clean and classy, but also dramatic.

~ The other color scheme I think would look nice is a watercolor mixture of pastel shades.  Depending on the colors and flower choices, this can have a wildflowery, country manor or cool contemporary look.

One other comment.  If your gown is white, you will probably want to use slightly cooler colors; if your gown is ivory, you'll likely be happier with warmer shades