If you are referring to a Run Stuffing MLB, I would say an agility, strength, and vision. With this type of build, your DC during your developmental phase will have to understand that you will be very poor in covering anyone, and ti use the Special Chart Assignments for MLB during passing situations.
With that said, speed will not do much for you, since a smart DC using the proper depth Chart, would place your LB at the point of attack or in a position that your high vision will allow you see the ball carrier, high strength and agility to fight off blockers, and high agility to cut over to the point of attack. You will further need the strength to engage the ball carrier (especially a true power back) head on, and stop it cold. As I posted before, I know of an excellent run stuffing MLB that lacks speed, but whose high strength, tackling, and agility make him a threat to anything up thd middle.