See Challenge # 124.
I'm guessing the idea behind this challenge is that we somehow marry some of the major ideals of the two blocks into an "whatiffy" fusion. That makes it a difficult challenge, because in order to fully satisfy it, I feel like I have to ponder a bit what that would mean. For example, I wouldn't be satisfied just taking a character from Innistrad and just calling him or her a Khan. I mean, what would that mean, in the context of Innistrad? Nothing, really.
Innistrad and Khans of Tarkir are a difficult fusion, and it's not just because they have such a divergent genesis. I mean, Innistrad was top-down "gothic horror" and Khans was built from the bottom up inspired by its block structure! How different can you get? Add onto that that Innistrad ended up with a European feel while Khans has a pan-Asian flavor and it becomes one of the weirdest possible set fusions.
To make things easier on myself, I guess I should just look at the surface aspects of the two sets and be pleased with what I come up with. Let's take the three most obvious aspects of each set and see where that ends up. Innistrad had DFCs and a focus on the graveyard because of its horror setting. Khans was a wedge set about warring factions led by the titular Khans, and had an asian flavor. So let's say that Khans of Innistrad possess all six of these qualities: It's a horror-inspired wedge set with a graveyard theme, an Asian flavor, DFCs, and five opposing factions who are constantly at war with one another. Oh my goodness. What have I done? That's too much. Let's cut the Asian flavor, since the name Innistrad doesn't go well with that; and let's stick to the allied tribes from Innistrad as the warring factions, and have their third color (the enemy color) just be a splash.

Here's a card from the Human faction.

I really dig Mourning, since it gives you the potential to discard the card and still use the other side, or pay the cost if the card gets milled. You really created an excellent way to use the double-faced card mechanic on non-permanents. Innistrad was fairly clean-cut as far as its "tribes" were concerned. I'd like to see how you'd splash black into this faction in terms of rate of appearance (getting whole cards, Mourning sides, non-double sided off-colour activation costs?).

Also, did Mourning double as a go at a pun due to the whole day/night frames on double-faced cards?

I really dig Mourning, since it gives you the potential to discard the card and still use the other side, or pay the cost if the card gets milled. You really created an excellent way to use the double-faced card mechanic on non-permanents. Innistrad was fairly clean-cut as far as its "tribes" were concerned. I'd like to see how you'd splash black into this faction in terms of rate of appearance (getting whole cards, Mourning sides, non-double sided off-colour activation costs?).

Also, did Mourning double as a go at a pun due to the whole day/night frames on double-faced cards?

Thanks for the compliments. :-).

I'd imagine the third color only as a compliment on cards, not getting cards itself. It could appear on multicolor cards or, for the humans, as mourning costs.
I didn't intend mourning as a pun, but it does sort of work as one. :-)

So Wrath of God costed at how field-wipes are in the present with Lingering Souls as the Mourning. It actually puts the opponent in a bad place when they try to Duress you.

Well, board wipes with an upside have been at 5 CMC recently. This is basically that.