I just finished playing Layton's Mystery Journey! It's probably going to be a while before every video of the walkthrough is public, so here's a little mini-review.
I liked the game's setup. It's a series of smaller, self-contained 1-2 hour mysteries. Each of the seven millionaires gets a mystery, and our three main characters each get a mystery. Then there's the intro case and the finale case. A nice little setup. Some of the mysteries had interesting premises, and they usually played out well.
There's only one case I outright dislike, and that's the Madame Doublee case. The solution to that case left a bad taste in my mouth. Some of the mystery solutions are silly, and the game is silly in general. I liked the tone of the game, but that particular case was ugh.
The finale case tries to tie all the other cases together, but it does so in an unsatisfactory way. I think the main problem is that two of the game's plot threads are left hanging.
1. Professor Layton suddenly disappeared a few months ago, and no one knows where he is. This gets mentioned several times, like it's going to become important in the finale. But no. Turns out, it's just a cheap excuse for why Layton doesn't appear in this game. None of the main characters from his series appear, either.
2. Katrielle's sidekick in this game is a talking dog named Sherl. He's probably the funniest character in the game. The prologue introduces him as a human who was turned into a dog, only now he has amnesia, so we have no idea who he is or where he comes from. Again, this seems like something big that the ending would resolve, but no. Turns out, amnesia is just a cheap excuse for why the game doesn't go into his backstory.
I would like a sequel, provided that it addresses these two issues. It could try to bring in the Layton Brothers spinoff game, if it was particularly ambitious, but I don't think that's entirely necessary. The game does throw a reference to Layton Brothers in a bonus scene that's unlocked, if you solve every puzzle of the game. Hey, that's something!