Ability to attach to a marine's back
The idea is pretty simple and I think that it has been recreated before either through plugins or in a different mod of HL. In order to boost the effectiveness of ambushing, a skulk could be given the ability to attach to the back of an ambushed marine. If the marine is alone, he would not be able to fight the skulk as it tears him/her apart. Bonus points for moving in a squad of at least two marines, where if the attacked marine can turn around properly then the team-mate can shoot the skulk.
This can also be extended to other lifeforms, such as the fade and lerk, who would also be given this ability.