Various mods by zahar0z

Who said that anyone is building a "creature concept" around this ability? It's just a bonus. Since there aren't many fire abilities in the game.

Then what's the point of adding an ability that will almost never be used? Very situational abilities can be found in creatures of level 6-7, but not in level 4 (I mean that, in terms of balance, level 4 creatures shouldn't have a lot of abilities in general, and poorly chosen ones can make the creature unappealing). If they were a replacement for regular golems, it might make sense because of the guaranteed presence of Archmages or Marsh Golems, for example. And there are enough fire abilities - vulnerability aura, liquid flame, explosion, explosion on death, etc. Although, as I already wrote, a level 4 creature would be perfectly fine with a shield + 50% damage reduction + slow immunity.

There's an option to upgrade all golems to tier 5.

Well, at level 5, you could even give them "Fire Strike" for a guaranteed chance to trigger the "Sign" skill. Although, copying the Jarls isn't a good idea.
This is actually a good idea if you want to preserve both features of the golems - slow immunity + damage reduction. You could try a shield + explosion on death + slow immunity + damage reduction.