Hi Niall,
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I see no reason why Flaws could not increase. I like the Greek-style sense of nemesis which tracks the Heroes as they increase in power. If Flaws remain static by default, then a Hero will automatically outgrow their Flaws over time. To emphasize the dramatic nature of the game, Flaws should stay proportionate to the abilities of the Hero.
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Which is not to say that Flaws cannot fluctuate. If a Hero acts in such a way as to reinforce their Flaw, then it should climb independent of the other abilities. This reflects a choice on the part of the Player, and can thus be reflected in the ability ratings.
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Likewise, if the Player devotes time within the game to somehow mitigate a Flaw, then the rating can decline.
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Both of these alterations are a direct result of actions within the game, possibly with story consequences attached. Yet, the default is the gradual increase in the rating of a Flaw to ensure it remains a problem for the Hero, and not just an easy source of extra points which naturally fades away.
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YGMV, as ever
Phil
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