There should be like an "Impostor Chance" that goes up each round if you're not imposter. The player(s) with the highest imposter chance(s) is/are the imposter this round. Once the round ends, the imposter's (or imposters') imposter chance is reduced down to zero to prevent back-to-back imposter games or never being imposter at all.
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Having the percentage to be an impostor go up in a bad from a game design perspective. It needs to be completely random or else the game falls down in the long run just like @anon1233 pointed out.
While being the Impostor is fun, it is also fun to beat the impostor by completing all the tasks on the ship.
I personally love not being the Impostor and ejecting them out of the air lock.