And this is where I would say that Oni God would refer to one of the devas.
No it doesn't.
Why the hell would we get another oni? *FACEPALM* We already have 2 of those, we don't really need another one.
WYNAUT MAK IT HAV FREE HORNS!?!!?! (In proper english: Why not make it have 3 horns, Suika had 2 and Yuugi had 1, so it could make sense.)
Well, even besides the fact that we know of an entire oni culture established underground and another culture of oni working for the Ministry of Right and Wrong, we know that we should have four oni making up the Big Four of the Mountain. Even assuming Kasen is the third, we have precedence and available room for the last. Personally I don't think this game will be that case, but we definitely have room for more oni.
But that really didn't stop ZUN from making Shou when we had Chen and Orin. So either another returning, or a new one.
None of those are the same type of youkai. Chen is a nekomata, Orin is a Kasha. Shou is a generic tiger youkai. But even if they were the exact same type of youkai it shouldn't be a big deal. Are you also perturbed by the fact that there are hundreds of kappa, hundreds of tengu, hundreds of moon rabbits?
I'd say ZUN is going for more clever wordplay here like he did with Shinkirou for th13.5. It looks a lot like he chose the kanji for the sake of making "kishin" work out instead of for the meanings of the kanji themselves.
I don't think it's apt to say that it was intended to be kishin just because that's what you see. We haven't even stepped into the guts of the game's plot yet, never mind what the game is actually about. "Kishin" can mean several things, not that many are used, but "shinjou" can have a pretty wide and varied number of meanings. But regardless, it still isn't necessary that he has a particular pun in mind at all, and I think it's much too early to say so.