I don't think kill stealing has to happen, just take the existing helping system where anyone can run up and help you kill a non quest mob and modify it so that, assuming your in the proper level range, you also gain some exp, no drops imo they should only go to the orig claimer.
I like the idea of ad-hoc grouping, hell mobs do it to us, why not let us do it to them in a more beneficial to everyone way.

On the exp note, a person joining this way should probably only get their base exp * mob_health_percent when they joined in the fight, so as to not have people running around seeing people fighting have a mob at 2% hit it once and get full exp.
So basically a mob that if I solo would give 300xp but I join someone else in killing it when it's at 30% I only get 300 * 30% = 90xp
And of course the level gaps would have to be followed, being a lvl 10 chasing a lvl 45 around to leech his kills would be an equally broken system.