The Tavern

Ignite poison is an interesting spell because of the way it interacts with mephetic and poison clouds, this offers great situational and positional play that really add depth to Venom Mage play.

There is however another side off ignite poison that I think is less interesting, it is in almost all cases the superior choice over letting poison do its work for very little downside. A very large portion of Venom Mage gameplay consists of repeatedly spamming Toxic radiance or poison cloud immediately followed by ignite poison. This combination is present in the starting book and is often optimal without much thought. In my opinion this dilutes the nature of poison of being a damage over time effect to yet another source of instant damage.

I suggest replacing the ignite poison spell with an ignite clouds spell. This keeps the interesting aspects of the spell while putting more focus on Poison magic as a source of damage over time.

Rebalance as needed.

Bonus idea for if this spell is implemented that would increase its power:

Expand the ignited cloud 1 tile past the source cloud, giving a combustion feeling.

If this change were made, I think the poison cap would need to be removed as well, unless the goal is to have players use Radiance to cap poison stacks, then flee while waiting for those stacks to slowly chew through the hit points and regeneration of sturdier monsters, before repeating the process (which would cripple Naga VM of Chei).

IMO Radiance is already limited enough in utility due to how many monsters are poison resistant/immune, that it doesn't need to be further handicapped by having it fail to produce any effect on monsters that hit the cap. As is, Ignite offers a way to cash out those stacks so Radiance can start doing work again, making Poison magic viable (albeit expensive) for standing your ground against groups of susceptible enemies.

I agree, some rebalance would be needed. I don't think the change would have to be that large though because of the stacking nature of poison a relatively small increase of the cap would already make it more powerful. I still think the change would be worth it to give poison a more focussed damage over time role compared to the gameplay ignite poison forces it into.