ToME: the Tales of Maj'Eyal

As the Necromancer is being revamped in the current beta I thought this would be a good time to suggest a change to the Impending Doom skill. This skill currently does arcane damage when never seemed to fit thematically. Wouldn't it make more sense to deal darkness damage?

Sort of. But you could use a similar argument to say that all Archmage Spells should do Arcane damage. And that would not result in interesting gameplay.

Thematically, Necromancer is about extending life via magically fuelled undeath. That doesn't automatically imply Darkness, and it also has Cold. Via minions it will be doing Physical and Blight as well.
Thematically, Impending Doom uses magic to make it so that the victim will soon die. It doesn't really go into any detail about how or why they die, they just do.
So thematically, you could go with Darkness and say that 'Evil Energies' are sapping the target's life away. You could equally argue for the use of Cold, Blight, Temporal or just a negative healing effect.
Or you could use Arcane, which means 'mysterious' or 'secret'. Which we tend to use for miscellaneous sources of Magical damage anyway.
So thematically, you could really argue for almost half of the damage channels in the game.

However, consider the gameplay. There are three aspects to look at.
1: Do you really want to lock anyone using this spell into having to use specific other categories? If you make this do Darkness damage, then it become favoured by Nightfall builds, and disfavoured by other types.
2: It scales with the target's life. If it was to share a damage channel with something you could easily focus on, that will mean that we can expect people to have a +50% damage bonus on this talent. That is likely to be a balancing nightmare, and will compound the first issue.
3: What about resistances? Some Undead automatically resist Darkness damage. Would it make sense for a Necromancer to have difficulty unmaking an Undead creature? Resistance Piercing bonuses are available, but having something powerful that is not on your main damage channel is quite useful at times. (Thankfully nobody has decided that all Undead should be resistant to Darkness, and that Necromancers should only do Darkness damage, or Necromancers wouldn't be able to get anywhere in the game.)

Fair points. And actually your ideas for cold, blight, or even physical I like better than arcane

Current 1.7 build has already changed this to deal Frostdusk (split Cold/Darkness) damage instead of Arcane. The description for the spell still says 'arcane', but this is incorrect and I'm sure it will be fixed.