I can't see those four things working together in one leg sleeve. I would guess (and it's only a guess) that the dragon turtle, because it supports things, would work well for the leg, and then the foo dog could be located higher up, on the leg, or better, on the arm, because it guards temples. I can't see them working with oni or hannya masks: they seem completely unrelated ( not religious symbols). They could work with maple leaves, representing autumn, and impermanence, but even here I'm very very hesitant to recommend maple leaves with immovable/permanent objects and ideas like the two you mention.
But you understand the complexity of all this. Part of the beauty of Japanese tattooing is having one or two ideas per body part. This seems to me to be simply too much. You should spread those ideas out over the whole body. Do you have a backpiece? I ask, because in Japanese tattooing that's the centerpiece, from which these other ideas are secondary.
But I'm by nooooo means an expert!!!!! I'd be very interested to hear others' ideas.
Where are you located/who are you thinking about for the artist?