First let me say, IMO most rules and / or laws do not pertain to 99% of the people. It is the 1% (or less) that dictate how the rest shall be controlled. That 1% can create tremendous havouc. Please keep this in mind for my discussion.
I was also hoping we could create a constructive forum, or brainstorm mode, to get friendly fire and collisions workably integrated into gameplay.
Treason is probably the biggest reason friendly fire is off. The TreasonDweeb

committing the treason will be caught, and appropriatly be hanged. (punnished or 86'd) Placing HTC employees in the postion of judge and jury, this is also IMO bad.
By then however; another individuals experience has been ruined, the damage done regardless. How easy would it be for this war crime to totally shut down an airfield at the spawn piont?
Incidental or accidental friendly fire is what I would like to see. IE: "Watch your aim you lose cannon, fireball casting Wizard, that hurts!".
To use friendly fire, the person doing the friendly fire would have to know he is committing it. This is NOT realistic, but do to the consquences that would be forced upon abusers of friendly fire, would be a requirement.
Friendly fire is VERY realistic, and would change many strategies within the game.
Runway spawning is probably the biggest reason friendly collisions are off. The issue of treason, also becomes a factor.
Realizing it is MUCH easier said then down, and probably already been done or thought of, my ideas for friendly fire and collisions are:
1) Set a timer, of say, 30 seconds after spawn or create a safe time limit.
2) A safe airspace bubble around the runways or create a safe zone.
3) Any friendly afflicted damage affects both the do'er and the done too, eye for an eye.
4) Drastically effect the players score in a negative manner.
5) A global announcement via the HOST "TreasonDweeb, flying for the Schmuck's, has just wacked a buddy".
6) Allow the offender to discover how frustrating Net Lag can REALLY be, delay all actions withing the arena, for a timed period.
I would like to see this work.

Mino