So the coin flip continues to be a thorn. But instead of strategems and increased mulligans for blue, what about a fundamental change to the way Gwent works. Eliminate the mulligan completely for Red. Give Blue one random mulligan and one targeted replacement (I.e you get to go into your deck and pick the card you want.) Let the winner of the toss pick which side they want. Red still gets the last say in the round which is very powerful, and Blue now has a functional advantage that makes it legitimately tempting.