There's an important work by a Very Distinguished German master of opera, which has plenty of females, but in my opinion no heroine.  And no mortal women, either for that matter

IGI - Yes!  Do you agree?  I can think of another Wagner work where the female lead, poor dear, is not the principal interest, but you might like to argue she is a heroine really.

There's a post Wagnerian work of immense high seriousness, which certainly qualifies.  (I don't mean ROSENKAVALIER, although you can argue Sophie's a wimp, and the Older Woman is not really the principal feamle lead, although definitely the star billing.)

Or Irene, Rienzi's sister?

IGI - I would hardly have the temerity to question you on matters operatic, but you wouldn't say ROSENKAVALIER has no hero just because the male lead is sung by a woman, so for the same reason, Curlew River has a heroine.  (And very moving she is too.)

There is a Britten opera without an obvious heroine - in fact the only one possibly that has male/female juvenile characters in love, but not central characters.  A seconda donna, (so she is a mezzo, in fact Nancy Evans at the premiere I think.)

Yes, I was thinking about Kundry when I mentioned Wagerian female leads.  You think she is, OK.

But post Wagnerian (and its about writers'. or at least composers' creative blocks as much as DEATH IN VENEZIA).  At ROH in the last fifteen years, but not regular repertory material. 

... and I guess this must be Palestrina?

If we can have COX & BOX, then I'd like to add Offenbach's THE ELECTRO-MECHANICAL SINGING LESSON, m'lud

William Tell? Mose in egitto? Viaggio a Reims?

Viaggio a Reims was what I had in mind.

The prima donnas in Moise and Tell are not essential to the plot, but they are there.

IGI - do you prefer Viaggio or Ory, as they use much the same music?  I think we are lucky to have both.