The main point of this post is to make it either more visually evident in game when enemies have higher or lower stacks of soulblaze, and also give a better explanation of what soulblaze actually does.
In terms of a player experience, I tried out some of the psyker feats that apply 2 or 4 stacks of soulblaze. In the training room set to Heresy, 2 stacks only does like 5 damage per tick. So I disregarded it as useless initially.
However, after trying out the soulblaze feat at 30 and the flamethrower staff, I realized that the stacks don’t add flat damage. You can keep on applying more stacks of soulblaze with the staff and the damage increases in a dramatically rising curve. The highest I’ve gotten stack applications before the thing outright dies is about 300 damage per DoT tick, applied on multiple enemies at once (thousands of damage-per-second in total).
My main suggestion is to make the visual effects (VFX) of the flames indicate how many stacks that enemy is burning with. This could be done via a hue shift, or more particles, or longer particle lifetime, really anything to visually set the enemies with high stacks apart.
This would provide players with engaging combat choices; the psyker could choose to continually stoke the flames of high-stack enemies, and the allies could avoid those targets to kill other ones (similar to how players know the Brain-Burst glow means that enemy is likely about to die, so they shoot elsewhere).
The secondary benefit to a varying VFX-per-stack would be that it would help indicate to players that the most important aspect of the mechanic is stack-amount. If soulblaze looked like little embers at 1-4 stacks, and then gradually ignited into roaring flames by the time it got to 10+ stacks, then the effect would be extremely apparent and satisfying to use.
Even if it isn’t viable in development to change the VFX as I’m suggesting, then at least more of a mechanically explanation is in order (but there are plenty posts about that kind of suggestion already - many mechanics are obscure).
It’s hard to to know when enemies have high soulblaze stacks, and it’s initially unclear what stacks do without extensive player testing.
Make soulblaze VFX be dim or dull at low stacks, but grow into larger brighter flames as the stacks increase.