The entire topic of (video) game clones is murky waters.
Technically no one can copyright game concepts or gameplay mechanics, which is why every successful game is sooner or later followed by copycats. Some are effortless clones, others have their own spin on it and even exceed the original.
A few things that are, however, under copyright (and possibly under trademark protection as well) are the names and all the assets. If the copyright holders of the boardgame were to find out about the Quoridor app then they could sue the developer as well as request Google to take down the game.
With board games, there’s also the often overlooked issue of rule books. Now, the rules aren’t technically copyrightable, but the texts in the rule books are, i.e. how they’ve written the rules. So if someone were to copy their game and rules while making sure that none of the names or assets infringed on the original’s copyrights, they’d need to find a way to write the rule book in a way that they aren’t inherently copying the original.
Now, even if your game wasn’t clearly in violation of someone’s copyrights or trademarks, they could still sue you. They might win, they might lose, but it’d be their right to sue you. As such, it’s often better to just play it safe and steer clear from such things.
Now, if you do decide to turn a board game into a video game, then the most important things that you need to take into consideration gameplay-wise are:
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Trying to keep the game as responsive and as fluent to play digitally as it is physically.
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Enhance the game in some way by adding something that the physical game couldn’t have, such as animations and automatic rule checking and other background processes (i.e. you can break the rules in physical chess, but you shouldn’t be be able to do that in digital chess).
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Don’t steal from others and break laws while doing it.