Ok i see, Seems my idea was already in the works

. Also Perhaps adding another Marine class.
Scout marine
Armed with a Scout Marine shotgun, Sword/Machete and a Bolt Pistol.
Maybe has a faster normal run speed than a marine, is slightly weaker and has a longer sprint speed. This is just so that he can get close with the shotgun do lots of damage then run out Ala Scout from TF2. It would be the offensive equivalent of a heavy, The Heavy is slow and does loads of damage, fast firing makes it perfect for defending locations, but when it comes to attacking it's slow speed makes it easy to ambush, So a scout would be more effective replacement in some instances.
A Addition to the Tyranids isn't a new class but is instead a new ability to the lictor.
If you gave the lictor the ability to break stealth and use its tongue to pull and drag marines, it would make game play as a lictor a lot more tactical, They could drag marines from the main group and then kill them and various other things. Making them a much better asset and more of a challenge rather than just a warrior that can cloak without the death spitter.
Thats all i've got for now.