I think supporting the general case of a mutually agreed off-book trade (aka "OTC trade" per the white paper) might make the most sense, assuming we will support all of the below:
using a limit order at a more favourable price to "pay" people to reduce your position
Q1: define use cases?
Transaction for off-book netting of positions
Alternative: a transaction for "willingness to do an off-book trade, up to a price, up to a given volume, against certain keys, or against all keys".