Well, let's see if we can simply it further)
I like dual damage types as well, but mostly because I am a bit lazy) and would prefer to keep swapping weapons to a minimum.
The proposed change to Firebrand, was mostly to:
keep it in sync with PoE1
avoid minor confusion about is it made of pure fire or not; is it a conjured sword (basked in flames); or is it really caster conjuring some fire blob and giving it the form of a sword. In later case it would make sense for the Firebrand summoning ability to have Fire keyword (and thus be even used as a counter to hostile Water/Frost DoTs and lingering effects, if there are any)
make the explanation of elemental weapons kw easier. E.g: all summoned elemental weapons have single damage type. Although if we'll go on a different approach it won't be necessary
For this reason, I don't think the Elemental Weapon Keywords should be any part of the solution. On top of the known vanilla breaking behavior, it most likely causes a lot more issues we've yet to discover.
I have checked the gamedatabundles, and if needed we could make those weakness effects trigger from Elemental Weapon Keywords too.
E.g. Shaken when attacked with Frost or Frost Weapon kw. And so on.
I like adding a second damage type to the already "elemental weapons" which are not so obvious for being elemental keyworded, like Frostfall, Frostseeker, Thundercrack, and the rest from your table.
But this won't fix the issue of getting "immune" messages when attacking with Grave Calling an Ice Blight.
Although yes, there definitely is a better chance for player to understand why he gets that "immune" when he sees that one of weapon's damage types is elemental.
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I have took a second look at what creatures do have elemental: weaknesses, keyword immunities, AR immunities, or healing. There aren't many:
So if need be, we can make them account for Elemental Weapon keywords too.
Interesting finds:
Greater Flame and Ice Blights don't have keyword immunity against Frost/Water and Fire; unlike regular and lesser blights. It's understandable that this is done such that they can be healed from respective elemental damage; but now they can be affected by effects regular and lesser blights could not
Greater Storm Blight still has Electricity keyword immunity; which kinda defeats it's HealOnShock passive (this is likely an oversight)
Some enemies with ConvertDamageToHealing have their AR high; others have it low or zero, in order to get more healing. Inconsistent, and will have interesting results with phys/elem weapons
Sand blights have no damage immunity, and neither elemental kw immunity
Dorudugan's ConvertDamageToHealing is a bit different from blights'. He converts all incoming fire damage into healing. While Greater Flame Blights convert all incoming fire damage from attacks with Fire keyword into healing (i.e. there also must be this Fire keyword on attack).
There seem to be only 9 different creatures that have elemental kw immunity, and these creatures are not that strong or important. A wild thought: what if instead of "Immunity" SE to Frost/Fire/Electricity/.. they had "HostileEffectDurationMultiplier" 0 vs these elements? Due to AR immunity, they either way won't take that elemental damage; and now they will practically ignore Frost/Fire/Electricity/.. duration effects too.
This would allow us to get rid of Elemental Weapons keywords; but still deal damage with weapons of dual phys/elem type.
Thoughts?
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And one more:
when attacking a Flame Blight (which has Fire AR: immune) with Firebrand (fire/pierce) - what result would you expect as a player?
when attacking a Greater Flame Blight (which converts incoming Fire damage from Fire attacks to healing) with Firebrand (fire/pierce) - what result would you expect as a player?