Well let's design it from first principles.
The arcade hack works by:
1. Issuing a challenge that is public on the arcade page (I think??!?)
2. Sending a pm to a user that they have been challenged.
3. Playing a game, recording the score.
4. The other person plays, scores.
4a. A PM is sent at this point???
5. The result is posted.
Anyhow, something like that.
Obviously a 'debate' challenge would have to be settled by humans, either mods or polls. Both would be nice.
I'm assuming only 1 vs 1 debates for now.
So here's the hypothetical scenario:
A new type of post, rather than post or poll, let's call it 'debate'.
User clicks 'new debate'.
Mr X makes a debate saying e.g "[Debate, Mr X vs Mr Y] Why starwars is better than startrek"
Similar to a poll field, a debate field would require the following options:
1. Who the debate is against
2. When a conclusion will be reached by (or never - like with polls)
Now here's where things could get more tricky.
On 'posting' this thread, it actually sits in the moderation queue. An automated PM is dispatched to Mr Y saying "you have been challenged to a debate on "Why starwars is better than startrek" by Mr X.
There the user can accept or decline the debate.
If they accept, the post is posted, e.g removed from the moderation queue.
Users debate as normal, and so can anyone else.
However after 24, 48 or whatever hours, defined when the user makes his debate post, a poll is automatically added saying "who won the debate? Mr X, Mr Y, Draw".
When the poll goes a week or so without a reply, the poll closes and a winner is declared.
Hows that for principles?