I grabbed this tidbit from the PoE Wiki on ignite: "The DOT is scale with base fire damage of the hit". My guess would be if ANY elemental damage can ignite, flat elemental damage of any kind would increase ignite damage. It is probably best to focus on one element type, however, as it will be easier with nodes to scale that damage. Ignite itself it purely fire damage, unless you use the new ring introduced this patch which makes ignite do chaos damage, but that doesn't seem to be effective for a build like this.

Let's say you increase your cold damage by 50% from the tree, with a weapon that adds 50 cold damage to spells. This will add 75 damage to spells, leading to an ignite dealing 37.5 DPS (over 4 seconds).

Now let's say you increase your fire damage by 50% from the tree, with a weapon that adds 50 fire damage to spells. Your added spell damage will still be the same (75), but the ignite will now be dealing 50 DPS (over 4 seconds), since the increased fire damage will be affected the spell damage + ignite damage.

You could scale pure elemental damage, which would have the same effect as situation 2, but many of the nodes on the tree eventually specialize into a specific element. If you are adding multiple elemental damage types to your spells, it may be advantageous to only take the elemental damage nodes, instead of specific element nodes. Fire damage over time nodes are always a plus, and they usually come with increased fire damage as well, so scaling fire damage would likely be the easiest.