Christmas form Pokemon have a battle entry effect with their snowflakes, and shinies have this glittering animation (and even a sound). I've always wondered why only the Christmas Pokemon have such an effect, and I'd love to see such an effect for more event Pokemon. Valentine forms could have a heart animation, Halloween maybe some sort of shadow, and Easter maybe flowers, etc. I'm not the most creative, I'm sure the community would have great ideas for this, but I'd definitely love to see more entry effects implemented!
PS: I'm sorry if this was suggested before, I couldn't find anything.