A bit more on this: Of all of these, only the Leukippos/Apollo/Daphne trio really
qualifies as extractable. The Rosenkavalier and Ariadne really require all of the
stage business (and people on stage) to work, and the Werther excerpts are so
short as to be kind of pointless for a concert reading - esp. as the first requires
the presence of the small choir of the Mayor's children, and the second is mainly
a scene for Werther, Albert, and Charlotte in which Johann and Schmidt have
only very small ancillary parts.

Do you have any flexibility as to operas you can choose scenes from. It seems
like what you're looking for are scenes with parts for Spieltenor in them. If this is
the case, you'd probably do better considering one of these:

Act I Sc 2 of Verdi's FALSTAFF, starting with the letter duet, and blossoming
into a brilliant nine-voiced scene (roles for you: Bardolfo, Caius)

Act II of Bizet's Carmen - the smuggler's quintet in Lillas Pastia's tavern (roles for
you: Remendado, Dancaire)

Act I of Beethoven's FIDELIO - From just after Marzelline's arietta through
Pizarro's entrance - a glorious canon quartet followed by a trio - or if you want a
full 20-25 minutes, start at the top of the act with the Marzelline/Jaquino duet
followed by her arietta (role for you: Jaquino)

Act III of J. Strauss's FLEDERMAUS - the prison scene following Adele's aria
(role for you: Alfred)

Act III Sc 2 of Humperdinck's HAENSEL UND GRETEL - From top of scene
through witch's incineration and and Hansel and Gretel's "triumph/liberation"
duet (role for you: Witch)

Act II Part 1 of Puccini's MADAMA BUTTERFLY - from Goro's entrance
following the Sharpless/Butterfly letter scene, through Sharpless' exit (or you
could include the letter duet (role for you: Goro)

Act II Sc 3 of Mozart's ZAUBERFLOTE - The entire scene (role for you:

Act I of Mozart's NOZZE DI FIGARO - immediately after Cherubino's "Non so piu
cosa son" through everyone's exit (role for you: Basilio) - or - Act III "Riconosci in
quest'amplesso" sextet (role for you: Curzio - whose only real function in the
opera is, in fact, to provide a tenor voice for this sextet)

Act III Sc 1 of Wagner's MEISTERSINGER, from either Eva's entrance or Sachs'
solo about being a cobbler (after Eva's "O Sachs! Mein Freund" through end of
scene) - just depends on whether you want to include her aria (role for you:

Act II of Leoncavallo's PAGLIACCI, from "Oh! Columbina" to the end (your role:

Sc 2 or Sc 3 of Wagner's RHEINGOLD - may be something excerptable
involving Mime or Loge (both roles for you)

Sc 1 or Sc 2 of Wagner's SIEGFRIED - may be something excerptable involving
Mime (role for you)

Act I of Puccini's LA RONDINE (role for you: Prunier)

Act II of Mozart's ENTFUEHRUNG AUS DEM SERAIL - starting after "Martern
aller Artern" - Pedrillo's "Vivat Bacchus!" through the final quartet (role for you:

Act III Sc 2 of Weill's DREIGROSCHENOPER - from Macheath's "Ihr Menschen
brueder to the end (role for you: Macheath)

Act II of Puccini's TOSCA, from Cavaradossi's exit to end of scene (incl. "Vissi
d'arte") (role for you: Spoletta)

Act II Sc 1 of Puccini's TURANDOT - the Ping/Pang/Pong scene - this would be
a REALLY good concert choice if you have 2 tenors and a baritone) (roles for
you: Pang, Pong)

There are also lots of possibilities in Gilbert & Sullivan. I'll leave it up to you to
research them.