Hmmmm...uuhh, McNinja, drugs 'er bad, m'kay?
Lol, but seriously... #5 is out the window ridiculosity
Number 4 would be feasible, because mechanically
PW attacks effectively equivalate to
MC attacks with this option. Though, powerfists would then be on same level of power, whereas they are supposed to be more powerful.
Third one would make more sense, but you've already got weapons that do that +2 soo...up those as well??
Second one, well considering that all assaults resolve against rear armor anyways in 5th, this is already in effect.
While number one is mildly complex, I see this one and option three as perhaps the more feasible choices going in-line with the descriptions of power weapons.
Well, those are my opinions on the five options you listed. Hopefully they're logically sound, but whatever...