I’ve often wished there was an Opacity parameter (with keyframes of course) built in the Text: Simple filter.
One case where this would be useful:
You could use multiple Text filters on the same clip and use the Opacity parameters to easily display/hide each one individually at any moment, with or without a fade effect.
I would be happy to see this as well. But if you don’t need a fade, I suppose one way to hide / reveal different text filters would be to keyframe the position to be off screen or on screen as needed - just have to change from on to off from one frame to the next. Hmm … something to experiment with …