I've seen this on other survey software and I would love to have it on limesurvey 3, too. I use a child theme derived from fruity and especially when having array questions, it would be very helpful and user friendly to have these expandable.
What I was trying to do is make ls-label-question for arrays clickable (and add a corresponding downwards-facing arrow behind it) so it shows or hides the corresponding answer-container. And as soon as an answer is selected, the corresponding answer-container is collapsed and the next answer-container is opened.