I wouldn't mind some sort of variation to lottery pop NMs. Changing it (or adding new NMs) that will automatically spawn after a set maximum number (won't get specific on numbers here) of kills upon the placeholder will result in the NM spawning. However, making it still possible (lottery wise) to spawn the NM before that maximum. Each kill increasing the chance of winning the "lottery" (other than the natural probability increase where if it hasn't happened yet, it is more likely to happen now) and spawning the NM. Eventually increasing to a point where the NM is relatively guaranteed to pop (a placeholder chain, if you will).
