I was playing Halo Reach with my little brother when it suddenly occured to me. Making 2 Varients for Both the Scorpion tank and Mantis
Scorpion
Varient 1: Remove the Cannon and replace it with the Spartan Laser.
Varient 2: Remove the Cannon and replace it with a Rocket Launcher that fires 10 missiles much like the Rocket hog.
Mantis
Varient 1: Adding a gauss cannon and remove the Missiles
Varient 2: Replace the missiles with a Tank cannon.
Another Idea is to give the Covenant a Mech like the Mantis. This would have a Mini-scarab gun for the primary weapon and a plasma repeater with boosted damage for the secondary weapon.