What if they put an upgrade on the reactor(150gas/min?) on a factory that makes it so one out of the two hellions that spawn get one mine, and you can plant it via the hellion and it does a tuned amount of damage/splash.
It seems to fit the role nicely - you can use them as an early defense and only one per every two hellions that come out and reduced dmg/splash allows you to lay more in a map controlling fashion with out being too powerful.