The Bane Homeworld, the Scruuge Homeworld, the Q-Pedd Homeworld, etc. appears when the season for each NPC arrives. Controller of the homeworld does NOT gain production, but instead gets an increased chance at seeing holiday NPCs. When the season is over, the planet fades into darkness and becomes automatically removed from all planet scans, and the final controller of the planet has it removed from his planets without GP abandonment cost.

Only drawback, it requires one open planet slot, just like any other planet.

Maybe following along the lines of your idea, during the holiday seasons special types of planets can be scanned and with each one that you colonize you increase your overall chances of finding that holidays enemy type by anywhere from 1% to 5% per special planet you colonize, and as you suggested after that holiday the planets all disappear until next year when you have to find them all over again.