This is for a homebrew Arcane Trickster subclass that specializes in fire damage. Although there are ways to bypass damage resistance like the Elemental Adept and Poisoner feats, there don't seem to be many ways to get past a damage immunity - other than changing the damage type using Sorcerer Metamagic / Transmuted Spell for 1 sorcery point.
Simplest option would be to state that your fire damage ignores resistance and immunity to fire - could do that if it's not too powerful, I'm just concerned that nothing else in 5e does that, it only substitutes one damage type with another.
My subclass feature substitutes the damage type for the purposes of overcoming resistance or immunity, but I'd like to know if it seems reasonable. The intention is that you pick one damage type when you hit 13th level, and that becomes a permanent substitution for damaging creatures with fire resistance/immunity - with the flavour that you're still dealing 'fire' damage, but it's intense enough to cut through.
Searing Flames
At 13th level, your flames burn hot enough to harm creatures that would normally be immune. Choose another damage type from acid, cold, lightning, poison, or thunder. For the purposes of overcoming fire resistance and fire immunity, your fireslinger spells that deal fire damage deal damage of the chosen type instead.
Another consideration is the interactions of the substituted type: a magma mephit has immunity to fire and vulnerability to cold.
So, Fire Bolt cast by a 13th level caster would do 3d10 fire damage. Target is immune to fire damage, so say you chose cold damage, Fire Bolt now deals 3d10 cold damage.
Should it trigger the mephit's cold vulnerability and deal double damage? I'm on the fence whether a way through immunity is enough of a perk already, without needing the additional benefit of triggering cold vulnerability.
Another monster to consider - an azer is immune to both fire and poison damage, so if you chose poison damage as your substitution, you're out of luck and can't hit the azer, at least not with Fire Bolt / now effectively Poison Bolt.
What about a balor - immune to fire and poison, and resistant to cold and lightning. If you chose acid or thunder, you'd deal normal damage. If you chose cold or lightning, you'd deal half damage, and if you chose poison, you're out of luck.
Should you be able to choose the substituted damage type each time? That seems too powerful/exploitable if you can gain the additional benefits of the substituted type, like vulnerability; but not too powerful if you can't get the additional benefits.
Should there be a cold to activate this feature - a spell slot, etc?
Another approach could be a straight substitution with radiant damage - might be overpowered.
Another another approach - as per the Searing Spell Metamagic from older editions: your spells that deal fire damage deal full damage to creatures with fire resistance and half damage to creatures with fire immunity.
This is for a homebrew Arcane Trickster subclass that specializes in fire damage. Although there are ways to bypass damage resistance like the Elemental Adept and Poisoner feats, there don't seem to be many ways to get past a damage immunity - other than changing the damage type using Sorcerer Metamagic / Transmuted Spell for 1 sorcery point.
Simplest option would be to state that your fire damage ignores resistance and immunity to fire - could do that if it's not too powerful, I'm just concerned that nothing else in 5e does that, it only substitutes one damage type with another.
My subclass feature substitutes the damage type for the purposes of overcoming resistance or immunity, but I'd like to know if it seems reasonable. The intention is that you pick one damage type when you hit 13th level, and that becomes a permanent substitution for damaging creatures with fire resistance/immunity - with the flavour that you're still dealing 'fire' damage, but it's intense enough to cut through.
Searing Flames
At 13th level, your flames burn hot enough to harm creatures that would normally be immune. Choose another damage type from acid, cold, lightning, poison, or thunder. For the purposes of overcoming fire resistance and fire immunity, your fireslinger spells that deal fire damage deal damage of the chosen type instead.
Another consideration is the interactions of the substituted type: a magma mephit has immunity to fire and vulnerability to cold.
So, Fire Bolt cast by a 13th level caster would do 3d10 fire damage. Target is immune to fire damage, so say you chose cold damage, Fire Bolt now deals 3d10 cold damage.
Should it trigger the mephit's cold vulnerability and deal double damage? I'm on the fence whether a way through immunity is enough of a perk already, without needing the additional benefit of triggering cold vulnerability.
Another monster to consider - an azer is immune to both fire and poison damage, so if you chose poison damage as your substitution, you're out of luck and can't hit the azer, at least not with Fire Bolt / now effectively Poison Bolt.
What about a balor - immune to fire and poison, and resistant to cold and lightning. If you chose acid or thunder, you'd deal normal damage. If you chose cold or lightning, you'd deal half damage, and if you chose poison, you're out of luck.
Should you be able to choose the substituted damage type each time? That seems too powerful/exploitable if you can gain the additional benefits of the substituted type, like vulnerability; but not too powerful if you can't get the additional benefits.
Should there be a cold to activate this feature - a spell slot, etc?
Another approach could be a straight substitution with radiant damage - might be overpowered.
Another another approach - as per the Searing Spell Metamagic from older editions: your spells that deal fire damage deal full damage to creatures with fire resistance and half damage to creatures with fire immunity.