For the sake of simplicity, we could make him Fire the RPG-7 depending on the angle of the Smash attack.
Low angled Fsmash = Normal Fsmash
Normal angled = Straight forward
Up Angled = Fires upwards(If Possible)
This sounds neat. I think the up angled one should be upwards at like a 30 degree angle or so. Making it too high up or straight up would be a bit unbalanced.
Pretty much, I see the RPG (forward) working like Samus's forward b; it shouldn't have deadly knockback and takes a bit of time before you can use it again to prevent spamming.
I guess the main issue is what happens when the RPG is charged up. Would it travel faster? Give more damage? Have a larger hitbox?