I think maybe you could improve on this sort of approach by weakening the invariants. Instead of trying to make sure that only valid "collations" get on the mai

I think maybe you could improve on this sort of approach by weakening the invariants. Instead of trying to make sure that only valid "collations" get on the main chain, let anyone willing to pay for it put collations on the main chain but define the first honest one on the chain to be valid. You can't tell which ones are valid at a glance but you can with O(shard history) resources.