well ... First of all I believe the stretching is not a specific power, in comics you can found very different quality of this power (Mr Fantastic, Skin, Morph or also the DC's Plasticman). I suppose that the worst problem on a power of this kind is the prepossession for grasp melee attack ...
How shoud it work ?
well I suppose it can work similar to Martial art with a stretching-character graphic: a "long" punch, a "hammer" hit, a smash defense, a whirling attack, a knockdown area attack, a smash and lethal defense, a loop attack, an extreme clash attack and a bind attack ...
I don't know in real I've never though about write it with all its rules but I can try if you tell me that it can be possible in game ...