Everyone else has them, so why don't we have them? Sure, maybe it's acid or something, but why don't nids have anything that uses a flamer template? Tyranids aren't so good at shooting, so wouldn't it make sense for them to have a weapon that doesn't depend on skill?
So, how do we fix this? Well, you could introduce a new weapon:
Liquifier - S: X-1 (max: 5), AP: 3, assault, flame template
It's profile matches something that already exists, namely acid mines, but without the vehicle killing (I blew a Demolisher up with acid mine last night, box cars on the pen roll, no real point to this I'm just bragging). I'd be willing to raise the AP if I could also raise the S to X (max: 6) or something.
Or maybe we could take an existing useless weapon and bump it a little. Say that the deathspitter has two modes of fire: one where the projectile, a fat grub-like creature stuffed with horrible poisonous liquid, is launched at a distant target, and another where the projectile is made to burst as it's leaving the barrel, spraying it payload in a short area immediately in front of the gun. It would have the same stats as a normal deathspitter shot, but would use a flame template instead.
So, could the Tyranids use a weapon like this? I could see small groups of flying warriors using these on marines entrenched in the woods. Would the change make the deathspitter useful enough to take, even with a bump in cost? Would any of this be unbalancing?