I think the title would come from 深い思い which literally means "Deep mind" but... as it, to me, sounds weird in English I'd rather say "Deep feelings/thoughts" Just saying. :3
I'm so glad they're coming with a new single already! The picture looks cool and it's going to be all dark probably, which is good. Their mini-album was great BTW. How can you not like Buono!?! *Pokes at Starra*