An incentive for actually living
Currently a skulk does not really have much of a reason to live aside to kill more marines or chomp more structures. Why not require skulks to have lived a certain length of time they must live that could be lowered by hives to be able to evolve into higher evolutions? Here's my initial time suggestions and rates:
Skulk Evolution: Instant
Gorge Evolution: Instant
Lerk Evolution: 1 minute
Fade Evolution: 2 minutes
Onos Evolution: 2 1/2 minutes
Mutation Evolutions (carapace, etc.): 10 seconds per evolution
Each Additional Hive: -30 Seconds
Obviously the times could be modified but the game would introduce a new incentive for skulks to live, as opposed to the popular idea of suiciding into a marine. Instead of thinking, "Oh I'm going to go onos as soon as the hive matures" the player must be thinking about his survival in his encounters and assess his ability to live through confrontation so that he may evolve into a higher life form, or go for the immediate benefit of attacking the 3 marines building the RT by himself in main in hopes of killing them.
The mechanic also creates more decisions for marines, there would be more of an incentive to finish off any skulks they might see retreating in an attempt to reset their timer.
The strategic effects of this, if implemented, would be a lessening in the overwhelming force of early alien dominance and a lowered mass of higher life forms in the form of rushes. Rushes and other tactics would still be possible, however, the commander and the alien team would both have to collaborate to ensure that they would have enough available aliens to perform the rush with the proper evolutions. It also increases the value of having a second hive.