There's a meme of sorts going around, or maybe two: one is to name ten pieces you never want to hear again (see Slipped Disc for about 120 examples), the other is to name ten pieces you want to hear more often.
I'm going to pass on the first, which would consist of overplayed 18th and 19th c. standards, La Traviata, Madama Butterfly, etc., and generalize a lot about the second.
For starters, you might take a look at my Fantasy Opera series. Most of the operas I listed are rarities of one kind or another. I'm sure Svanda Dudak is performed more often in the Czech Republic than outside it (although I see that it's being done in Palermo shortly).
After that, really, it's all new and old music. I believe I have never heard a Dufay mass performed live, and there is a huge wealth of music before 1700 that I would love to hear in person. Of 20th and 21st century music, I'm interested in modernists such as Crawford, Carter, Boulez, and Babbitt; Europeans such as Haas and Birtwistle; Feldman; women whose music has been overlooked or ignored; etc. I'm interested in music by the second- and third-string composers; the better-than-competent composers whose music is buried under the top layer. I'd like to hear almost everything that's off the beaten track.