What would be the Difficulty for a Spell intended to set a target on Fire? Not blast them AND set them on fire, just burn them.

Attack spell starts at D and adds +D for the Fire effect, so if we take away the initial base damage of the attack and just leave the Burn effect we have a single D spell?

What if we wanted to guarantee the Burn quality triggers without the need for Advantages? Bump the difficulty by one, or two?

Why not create a spell that do exactly what you it to do ? In this case just make a spell with the only effect is to ignite its target. No need to make it more complicated if the only thing you want is a spell that set a target on fire.

@bharrington73 you could just decrease the difficulty by 1 to to leave out the base but it gets a bit wonky to activate burn without a actual hit.

I would suggest you make something that is not an attack spell but uses conjure. Conjure a fire around the subject so base is 1 purple, size of the target if its a humanoid is 1 so medium summons adds 1 purple and you would need to add range accordingly. Since you are sorta doing a thing it was not intended to you might add 1 more purple but ill leave that up to you.

The rules for Fire you can find in the GCRB page 111. I would suggest the rating of the fire is = ranks in knowledge (what ever you use in your setting)

This would mean the target suffers rating in damage each round they are inside the fire. In the GCRB it says "Fires burn for as long as they have a
fuel source" which in this case could call for a concentration maneuver. You can roll out of it with a hard or easy coordination check depending on the circumstances or you could opt to use the standard Hard athletics check what ever works for you.

IMO this is easier then trying to hack the attack spell but but things could work.

I'm curious to know the context of this question. When you say target, do you mean an opponent, or do you just mean on object the player wants to ignite? Because the context and the target change the answer.

If you are in a structured encounter and the goal is to cause damage to an enemy, then you should treat things as normal and describe the attack as a fireball or burning hands or whatever (Just because the attack's Burn effect didn't activate doesn't mean they didn't successfully cast a fire spell). If the goal is to start a fire within the environment of the encounter, no matter what kind of spell you decide to call it (Attack, Conjure, or Utility) the base difficulty is Easy ( D ), and it's up to you if there needs to be an increase for the Burn effect (personally, I would argue that attack rules as normal still apply, and it's up to you what the wound threshold of the object is if you think the flame is powerful enough to consume the object before the encounter ends).

On the other hand, if this is an unstructured encounter, then this is a perfect place to use Utility. Base difficulty is still Easy ( D ) and all you need is one success and the target is on fire. If they want a huge spectacular flame, maybe increase or upgrade the difficulty to whatever you feel is appropriate. Now, if a player in an unstructured encounter wants to set an NPC on fire, it's time to roll initiative and move to a structured encounter (see the above paragraph).

Bottom-line: don't over-complicate it. If things seem confusing or you're looking for a mechanic that you can't find, there's a good chance that the effect or result you're looking for is a narrative result, not a mechanical one.

Specifically, this is an attempt for a Damage over Time spell. More generally, I'm wondering on the appropriate increase in difficulty necessary to remove the Advantages necessary to trigger a Quality.

I'm a bit confused on the logic here. Because the Burn damage over time is based on the base damage of the spell, why don't you want to guarantee the damage at least the one time (when you hit)?

If you want to be more sure of triggering the Burn effect, then take a manoeuvre or two to aim (and grab them blue boost dice) and use a magic scepter implement (adds a blue boost die to all spellcasting checks using it). That's two or three blue dice, which should give you enough advantages to trigger Burn.

Or have a chat with your GM and workshop a talent that works like Grenadier (spend a Story Point to trigger Blast).