My girl, if you're looking for good mezzo opera/operetta songs, you have hit the motherload. Try this site (The Gilbert and Sullivan Archive):
http://math.boisestate.edu/gas/index.html In each G&S show, there is at least one leading mezzo role (they often wrote with specific performers in mind for roles, so there is a bit of a trend in terms of the types of characters/voice types that appear in the repertory). Look into some of these roles:
Ruth, "The Pirates of Penzance"
Katisha, "The Mikado"
Buttercup, "HMS Pinafore"
Dame Carothers, "The Yeomen of the Guard"
Lady Psyche, "Princess Ida"
Mad Margaret, "Ruddigore"
Hope that helps!