Aya length delay is great for repetition of the aya, and having "next aya length" delay would be perfect for testing our memory and revising. So we'll listen to one aya, then once it's done we're going to recite the next aya throughout the delay and check if we were right once it starts playing.

Why should this feature be added?

Because some verses are short followed by long verses and vice versa, so having "aya length" delay would make it inconvenient to check our memory of the next aya in some cases. And having a 30 seconds timer could be too long for reciting some verses, while the 10 seconds delay might be too short.

So having a "next aya length" delay would make it just right.