In Mutant Crawl Classics RPG, there is a rule on page 42 stating that a mutant character may choose to re-roll a passive mutation upon gaining a level and may burn Luck or use glowburn in the process. In practice, this gives a slim chance of improvement made somewhat greater only at the expense of Luck, which does not regenerate for mutants or manimals, or physical abilities (Strength, Agility, or Stamina), which regenerate slowly and probably not in time to prevent a character from suffering severe disadvantages during an adventure. In addition, there is a significant chance that a character's passive mutation will worsen.
The question I ask myself when I think about a rule is: Is this fun? ("Is this fair?" is another question I ask and is often linked to the first.) Is it fun to survive long enough to achieve the next level just to see one's passive mutation devolve or see it improve at the expense of one's general ability to survive? It seems contradictory. I can understand the risk involved in radiation exposure, which might be to the mutant's benefit or detriment, but that risk is part of the adventure itself, not a rule pertaining to levelling up. It wouldn't be as much of an issue if mutants and manimals could at least regenerate Luck even if it were only 1 point per day. For me, this rule is neither fun nor fair.
So, here's my house rule...
Each time a mutated character gains a level, the player may opt to re-roll a passive mutation. If the roll exceeds the current result, the character's passive mutation improves by one step. If the roll is a 1, the passive mutation regresses by one step. Otherwise, the passive mutation remains unchanged. Neither Luck nor glowburn may be used to alter results.