Guild Wars 2

ArenaNet seems to have learned nothing from GW1. No one uses weapons with +X% damage against [Monster type], unless maybe it’s one particularly prevalent monster type in a particularly “hardcore” endgame zone. Which of course most ‘slaying’ sigils aren’t meant for. Weapons upgraded with those are way to situational, take up way too much space in your inventory, and if you are hoping to gear up in a certain skin/stat you are out of luck because you’d have to purchase and drag with you half a dozen copies of that skin/stat.
We already had that in GW1 and it didn’t work, except possibly for farmers. However, alongside that there also was a system of damage types. I.e. fire damage worked better on Ice Elementals, piercing damage was less effective against skeletal enemies, etc. Basically, look it up here if you don’t know about it.
http://wiki.guildwars.com/wiki/Damage_type
Now, I’m not saying this system should be transferred 1:1 to GW2. But I can imagine, a marriage between the two. For example, a “Holy/Light” sigil that works against Undead, Ghosts, Demons, and other less-obvious enemy types. Or a sigil “of the burning Heart” that is more effective against Svanir, Ice Elementals, alpine creatures and amphibians. Overlap would be possible as well, such that a “Judgement” sigil could apply to Ghosts, Renegades, and Outlaws equally. (Of course this would also mean that the dungeon-specific sigils would have to be revisited.)
Something like that. Essentially a slaying upgrade that works on foes found throughout the whole world equally well, not only on a certain enemy faction whole but also singular subtypes which are dictated by common sense.
Avoid that it becomes a prerequisite, but make it much more exciting to actually use such an upgrade, both for the damage-increasing aspect and for the “thrill” of discovering that your weapon is especially effective on that random mob.

Edit: I said “equally well”, but for balance purposes it could even be implemented with variable damage boni for each enemy type it works on.

I love it when an underwater weapon drops for me and it has something like “… of ogre slaying”. I have yet found any underwater Ogres. Where are my underwater ogres?

Damage types were also largely ignored in GW1. Elementalists went with fire because it has AOE, making it more effective in almost all PvE. The only time I can think of it not being the best choice was the last mission in Prophesies. Do you know what eles did there ?
They demanded a ranger in the party bring winter. Now all elemental damage is ice damage. Guess what the Titans were weak to.

Other casters didn’t have any choice in their damage types. As for martial classes, their choices are:
– Stick with physical damage and attach a useful prefix like vamp or furious.
– Go /e so they can grab an elemental damage prefix and use attunement. But to be able to switch damage types, they need 3 weapons identical except for the elemental damage they deal.

I don’t see how your suggestion would have practical effect to damage types or “of x slaying” mods. Players will still massively prefer the sigils that are effective against everything over the ones that only work on some enemies.

Grouping racial damage would work much better. As they are, those sigils are only used in the mystic forge for recipes, hoping to get a better one like Bloodlust.