Couldn't you make it: "If two creatures would fight, instead choose one. That creature's controller sacrifices it." That's not too bad.

How about ": The next time two creatures would fight, they fight an additional time," possibly with "Any player may play this ability".

That seems to cover the "on and on until one dies" concept. I considered just saying "they fight until one of them dies" but then two 0-power creatures would cause the game to go into an infinite loop.

Surely the point in this card is to have wording that sounds like it's off a movie poster?

Well yes - there's lots of ways to do it using lots of words. But I'd really like to have it be this short.

This would only trigger after the fight takes place, so if one of them takes lethal damage, this causes it to take another creature (of your choice) with it. Possibly the other one involved in the fight, but the way I'm reading this there's nothing preventing you from choosing a random bystander (or even a Blightsteel Colossus) to get caught up in the carnage.

When two Elephants fight, put a -1/-1 counter on all Plants.

Maybe I'm Just Like My Father
­
Sorcery
Put a 1/3 white Bird token into play named "Dove". It may Fight another creature.
If two Doves fight, destroy all other creatures (Alone in a world so cold).
Why do we scream at each other?

Watermark: The Symbol formerly known as "Watermark".

SM's issue could be fixed by making the wording "Whenever two creatures fight, one of them dies". He also alludes to the way that "dies" (as a shortcut for "is put into a graveyard from the battlefield") has never been used as an imperative before, only in conditional clauses. The more normal version would be "destroy one of them", but that loses the movie-poster factor.

­Arena could very nearly be worded "Choose target creature you control and target creature of an opponent's choice he or she controls. They fight."