Nodes could delay relaying of blocks including potential double spends (txs replacing txs the node knows about).
I would guess that major miners already are, or will, at some point, communicate directly among each other. After all, you don't want to miss a block.
It is not clear to me if one could not have a convention to under which miners might ignore certain blocks though.
I would expect the large miners to be well connected, too. Will this still be possible with the new protocols? Will there be mining supernodes with several thousand miners connected to share blocks as fast as possible? Might well be possible.