What do you mean exactly as concept album?
To be really precise, they NEVER did a real concept album, in fact:
SFAM is not at all, is a rock opera instead;
SDOIT disc 2 is a suite, which if left alone would have been a concept album, or sort of (usually CA are not one single suite), but then we have disc 1 which has nothing to do with disc 2, then no.
Octavarium is a concept only in musical terms, but the concept in terms of lyrics is simply non existing.
So if you are referring to a first-time-ever concept album, as they already covered extensively mental disorder, which is very intriguing although not new, there are some ideas they could work on, eg being # 13 an album with songs inspired by premium horror/therror stories (various writers, one novel for each song), but it's not us deciding on this, guess JP drives the whole thing, as always.
Same if you mean sonething like SFAM, so not a CA but a Rock Opera (I already covered this in another topic). This would really depend on JP inspiration by reading a book or watching a movie which sould inspire him... Something about science-fiction would be cool. And what about Kafka? Inventing from scratch is a huge risk...