Darkvision and stealth... and movement order
 

Darkvision: does not affect destealth, according to the manual. It arguably should, given how it would affect the ability of others to take advantage of the cover of darkness. Likewise, darkvision on stealthy troops should be associated with higher stealth -- it's easier for them to take advantage of night and not blunder into a patrol.

One might also conjecture that it would be useful for an army, in terms of being able to steal a march on an enemy -- but this makes more sense if tied to the currently non-existent ability 'need not sleep'.

Re: Darkvision and stealth... and movement order
 

It depends on how much stealth is slipping by at night (eg fiends of darkness), and how much is bushcraft/magic (eg all the pangaea types), which would make it tricky to mod.

Undead get a "need not sleep" movement bonus, I'm not sure if any magical beings do.