I'm not sure if this has been mentioned before, but would it be possible for sentences to provide multiple audio files in different regional accents?
For example, one audio file in Peninsular Spanish, another in Argentine Spanish, perhaps another in Mexican Spanish...
Or one in U.S. English and another in British English, and another in Australian English.
In cases where the sentence is natural in multiple dialects (which happens quite often), I think this would be beneficial to learners, even if it would obviously depend on the willingness of people with different regional accents to contribute audio.
If someone is active on Forvo, then it might also be fruitful to try developing cooperation with them. The website mostly contains pronunciations of words, but also some sentences. There are often several pronunciations and the region of the speaker is displayed.