Arsonists are a type of rogue who have a penchant for fire. They delight in making little flames where ever they go, and watch with glee as these tiny motes quickly grow into infernos. Combine this destructive habit with a little bit of magic, and you have an agent of chaos, sure to sow embers and mayhem anywhere their dark little hearts desire.

Fire Starter

Starting at 3rd level when you gain this subclass, you gain a knack for making fires. You can cast the Produce Flame cantrip, and can use your cunning action to cast this spell, or to hurl the flame. 

Blazing Communal

At 9th level, your obession with flame sears your words. You gain the Ignan language if you did not speak it already. 

Flame Imbuement

Also at 9th level, your weapons reflect your desire to cause fires. When you make a sneak attack, you can change the damage type to fire. This fire damage ignores resistances to fire damage, but not immunity.

Smoldering Visage

At 13th level, your destructive impulses literally come exploding out of you. You can, as an action, produce smoke which comes billowing out of you in a 10 foot radius. This smoke blinds creatures who start or end their turn inside of the radius, lasting until they leave it. You are not blinded by this effect. This effect lasts for (1 + your dex. mod) rounds or until you dispell it as a bonus action. 

Burning Body

Also at 13th level, your blood begins to boil. When a creature hits you with a melee weapon attack with a non-magical wooden weapon, it ignites. Weapons that are ignited in this way take a permanent and cumulative -1 penalty to damage every round until it is put out, but also deal an additional 1d4 fire damage on a hit. If such a weapon takes -5 to damage, the weapon is destroyed. Nonmagical wooden ammunition which hits you is destroyed instantly. You can activate or deactivate these effects as a bonus action at the start or end of your turn.

White Hot Assault

At 17th level, you become so well versed in the art of making fire that it transcends physics. All creatures that would have immunity to fire damage instead have resistance to it as it pertains to your attacks. Additionally, when you roll a critical hit against a creature or object, an explosion of flame that is functionally identical to a Fireball cast at 3rd level appears centered on the creature you attacked. 

Herald of The Conflagration

At 17th level, you have at last claimed the title as a pure being of fiery mayhem. You become immune to non-magical fire and gain resistance to all other fire damage. Additionally, you may summon a fire elemental, as if by the Conjure Elemental spell, except it does not require material, somatic, or verbal components, and only requires an action to cast. This can be used once per long rest. 

Some thoughts: it's a bit top heavy, with multiple benefits at higher levels where most rogue subclasses only have one 9th, 13th, or 17th level feature.

Fire Starter - awesome, I like it.

Blazing Communal - could be a 3rd level perk.

Flame Imbuement - seems powerful to gain sneak attack fire damage and the ability to bypass fire resistance in the same feature.

Smouldering Visage - how many times can you use this per day? It could effectively give you advantage/sneak attack on foes who rely on sight within 10 feet of you, since they're blinded and you're not. Also, is the area considered heavily or lightly obscured for those outside the effect?

Burning Body - does it only affect entirely or majority wooden weapons like a club, or does it also affect weapons with a wooden handle like axes, halbards, etc? Does this feature completely nullify wooden arrows and crossbow bolts?

White Hot Assault - this is similar to the Searing Flames feature of my Rogue Fireslinger subclass. I think ignoring fire immunity outright is possibly too powerful, so I made it limited use. The free Fireball on Critical Hits seems a bit much - if it's a crit Sneak Attack, you'd need so many d6s!

Herald of the Conflagration - I think you're doing too much at 17th level with two benefits from White Hot Assault, and two more here. The two level 17 features could be condensed into something simpler.

I wonder if Burning Body could function more like you can cast Fire Shield (the warm version) once per long rest.

And the 17th level feature could give you a casting of Investiture of Flame once per long rest.

A more fundamental question is how does it synergize with being a rogue (same problem as I have with the fireslinger)?

Thanks for the imput! As for that last question, I’ll be the first to admit that synergy wasnt at the forefront of my mind.  
I didnt even know that there was a fire related rogue subclass already!