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Leo's suggestion about skipping a turn is quite reasonable and worth discussing.
But, honestly, the most sensible suggestion regarding save-loading that I've heard is this: the number of save-loads is unlimited, but they must be taken strictly in turn. That is, any player can request the first save-load. This automatically gives the right to the second save-load to the next player. No player can take two save-loads in a row, but if a player takes the first, then the second, the first can take another one. And so on, endlessly, even if each turn involves loading the game 10 times, as long as it's strictly in turn. This way, you'll always be protected from a single random mistake; the first save-load will be built into the game rules, but your blunder will give your opponent a chance. And an unused save-load, i.e., if your opponent has taken one more save-load than you, can be converted into a turn skip, according to Leo's system. In my opinion, this is optimal.
As for "strictly at the discretion of the players," I've never liked this option because there are players who simply don't allow save-loads. And if you lose a game because you were too lazy to check the portal, even though it's frustrating, it's at least somewhat justifiable. But I consider a loss due to a misclick to be completely unfair.