Puck never does sound quite right to me--nor Ariel. They sound too human. (The animated Shakespeare version of Tempest tinkered with Ariel's voice but didn't do a good job.)
It would probably annoy a opera singer to digitally manipulate his voice(*), but for some special characters the effect might be worth it. Suppose you distort Puck's voice in the direction of pure tones--but not too much or you lose the syllables. It would, if not overindulged, make it sound more alien but still recognizable.
Puck and Ariel are dangerous. You might want to add a fainter parallel version of the speech dropped a couple/three octaves. (faint enough to add flavor without interfering or distracting)
Why yes, I think opera lends itself very well to the movie screen, if the sound system is good enough. When the music is continuous you can't split the scene into multiple takes, but everybody can get a good rest in before the next section, the viewer can be in the action much more deeply, and you can do more thorough miking. Plus you won't giggle at Siegfried's dragon.
(*) Pop singers must not care so much, if I can take the widespread use of autotuning as an indicator.