For a subject like abortion it would probably be appropriate to have teams of 2 people for either side of the argument.

The debate I think should be restricted to these teams which would have to be decided beforehand. Otherwise its just another thread. The thread debate is closed to posting and only a moderator or admin can add the posts. The posts for each side should be forwarded by PM to whichever mod or admin is moding the debate. Opening arguments or rebuttals can only be posted when all parties have forwarded them to the mod so no advantage is given.

I don't think any more formal structure should be imposed for the moment although this can be refined over the first few debates as issues may arise.

I think after one or two rebuttals, then a closing argument from either side with bulleted list of the main points of their argument.

The separate discussion thread can be the form for other members to decide who has won the debate. Perhaps a poll added to the thread to that effect.

Always remember with debates that the winner is not always right. The winner is the one to provide the best argument. Some of the best debaters I've seen have won arguments based on the opposite of their personal views.

One experiment I would like to try eventually is to get two members to volunteer to a blind debate. The subject of which and the sides taken are picked by other members after the fact. This is not intended to put anyone in an uncomfortable position but rather to experiment with creating arguments for a debate which don't particularly reflect your personal views but convince the other members that it does.