From what I realized by tests, we can reset the global_settle flag, but due to the fact that we cannot remove the settlement price, we cannot yet reuse prediction market assets.
I would propose to have the settlement price expire similar to price feeds by a definable parameter .. after that time, the asset should be re-useable to other prediction marktes if the global settle flag is reset.
before the expiration of the settlement price, the issue could withdraw customers unsettled assets, settle them for them and return the earnings (if any) minus fees or so.
But we certainly need to update the graphene base for this. Not sure if it requires a hardfork though, I assume we do!