Well, nobody uses the rocket launcher as their primary weapon, right? There are situations in Doom where the ammo god has forsaken you so bad you're going to fire 70 rockets in a row because you have nothing else. For each other weapon, there's a specialisation trait structure. Dualgunner/Gun Kata (which I always delay because Eagle Eyes and Finesse are more worth it imho), Ammochain, Army of the Dead, there's even two different possibilites for close combat (Blademaster/Vampyre), but there's nothing that really enables good rocket launcher play. Fireangel, yes, but somehow I never use that because it wouldn't be worth it.
In my opinion, it would be an idea to extend Fireangel to a rocket launcher game trait. My idea would be this: If you hit enemies with a rocket, the number of damaged enemies is counted, as explosion damage damages multiple enemies. The number of enemies hit should then give a damage bonus, such as a linear +10% per enemy, for example (Or encourage slaughter by making the bonus grow exponentially, such as: +0% for one enemy, +10% for two, +25%, +66%, +100%, +150% or something like that - please don't nail my idea because of the exact numbers, they are just figures to point out what I mean.).
Also, how about a trait that increases the explosion range on rocket launchers and BFGs, which would then be needed for even getting Fireangel?
I don't think rocket launchers would become seriously overpowered because, as I imagine it, a rocket launcher player needs to have a secondary weapon which he can not obtain master traits for.
Also remember that this trait would not make the Cyberdemon any easier.