Last Scenario/WMG

Ethan does not physically require sleep.
We know that had physical effects on Ethan's body, although, aside from turning his hair black, we don't know what these were. There are a number of points during the game when Ethan seems to be awake in the middle of the night, from the times he sneaks out at night near the beginning of the game to at least one time when you find him awake and staring at a wall (if you talk to him, he'll say that he'll be fine without sleeping).

Save points are justified.
As the Fridge Brilliance entry states, the researchers in the biorite facility were clearly experimenting on save crystals. They're also the same colour as biorite and give off a glow. Maybe they're actually crystallized biorite?

Hex tiles are pieces of a being's life energy, soul, or other part of Soul Anatomy.
If you do Saraswati's sidequest, you'll find that, so the idea that there's some link between the being and the tile isn't entirely unfounded. Also, often, you find a Hex tile after you just killed something, monsters in particular, though there's also Wolfram's tile, which is in the exact place he died, and Tazar's tile, which is also in the exact place he died. Coincidence? I think not. There is, of course, the matter of tiles that are of beings who are still alive, (Tiamat's in particular -- where did Barasur get it?) but I don't really have a good theory for those.
 * Possibly, he got Esmerelda's hex tile as a special gift from her. Maybe, among the Havali of old times swapping the hex tiles of each other was a ritual similar to wedding rings?
 * This ritual must have deranged and have been common exclusively among Havali, though. Random Hex players have Lorenza's, Thorve's and Augustus' tile, Valentin gives Matilda's tile away without a special reason, Eugene, the fat townsperson of Whitelake, the first Hex player you can encounter, has the tile of Hilbert's sister, the tiles of Hilbert and Ethan are lying somewhere in dungeons... and so on.
 * Maybe Hilbert's tile was stolen by Castor, who dropped or lost it a bit later, Ethan's and Lorenza's tile were stolen when they were trapped in Biorite, and Julian found Augustus' tile after ?
 * For Joanna, she could have seen her hex tile as "just a playing card" and given it away to the first Hex player who asked for it.

The Black King is Valkiris.
The Black Mausoleum is not reachable with any physical method of transportation, and it would make sense if it was created by the planet in the same manner as the Gate of Elysium. While the Black King is kind of an evil-themed boss, that is fitting, as he is undead (unlike the four heroes) and seems to have been beyond normal morality during his lifetime That raises the new questions who the Black Rook and Queen were, though...