This has been mentioned before, in a way, but not in any way where it would work well or make sense.
I think it would make sense for the Skulk and Lerk bite to take a small percentage of health when biting a marine. Like if you had 60/0 health/armor as a Skulk and you bit a marine, you would have now maybe 65/0 health/armor (Value subject to change). This would work on damage percentages so Focus could be used to enhance this innate ability, thus making Focus better for late game when marines have full armor and so on.
It would make some sense if you think about it. I mean, if I was a Skulk biting some poor blokes shin, I'd eat it. If I was a Lerk, seeing a lone marine wander around, I'd swoop in, grab a piece of his shoulder, fly away eating it. Yum.
Unbalanced or not, this wouldn't make too much difference to a shotgun blast, but it would be helpful for those escape situations.
please don't hit me