Ataksuki was where I got the idea, but it's more than that. What he looks like isn't important, but finally showing it after their obligatory monologue would be like the author saying there is nothing left to say, we now know all his secrets and as a character he's run his course. Even just leaving the battlefield like that felt ominous.
I agree. We can pretty much imagine Kakashi's face, considering we saw his father's face clearly and they supposedly look a lot alike. But it would be heavily symbolic if Kakashi lost his mask during this upcoming encounter with Obito. Whether he dies or not, finally seeing his face would be the visual complement to exposing the last pieces of the Kakashi puzzle.
Who knows -- maybe he has a "I

Rin" tattoo on his face.....

On the other hand, there is no obvious plot element that would be satisfied by him dying. He's lost people close to him. Usually, Kishimoto has Naruto go through things like this to force Naruto into seeing his opponents point of view, but Neji already died for that. Originally, I thought Obito was being used in this manner as well. Moreover, the whole thing just flops with Obito in a way it hasn't with others because of the method Obito and the basic fact that it requires Naruto to turn the world over to the guy who is inflicting pain on it. I don't think he needs to understand Obito to effectively call him out. Obito is so stained with blood that its really hard to take anything he says with a straight face.
As always, it depends on what Kishimoto has in mind. I agree that Naruto has already lost people, and as
tricksie pointed out Tsunade's death makes more sense. Theoretically the lesson has been learned, but has it really? Naruto doesn't seem to have grasped the point Obito is trying to make -- whatever that is. That's why I question if there couldn't be more Obito-related loss in store for Naruto.
Maybe I just got too sold on the possibility that Obito would realize that Sakura is Naruto's Rin. Kakashi's death -- or at least the serious threat of his death -- would impact both Naruto and Sakura in a way that might make their relationship more obvious and trigger something for Obito. Or he already knows (thanks to his spying) and that's what he's driving at. I don't know.
Like you said, the way Kakashi left the battlefield and essentially transferred leadership to Naruto was ominous. I've always had a nagging feeling about the other dimension, and about Kamui itself -- about if it might not end up that Kakashi and Obito to somehow cancel each other out and one or both remain stuck in the other dimension. There's been some groundwork for something like that, but it's not exactly clear if it's technically possible.
Yeah, the technical rules to how the Sharingan works aren't clear. Where have I heard that before?