I imagine most of us are familiar with the phenomenon where you need a specific mod as part of your core build strategy, but level up 30+ times and burn through all your rerolls without ever seeing that mod offered even once. This can be a... frustrating... experience, and will become more common if new mods are added to the game. To alleviate this, I propose a variable weighting system, where each time a mod currently in the pool is not offered, it gains a temporary, cumulative boost to its chance to appear. When the mod eventually does appear, it resets to the base chance. If it's not chosen, it'll start accumulating a boosted chance until the next appearance, and so on until it's eventually acquired or banished.