We already have some win effects, and in my opinion enough. If you get bored of the same one, change it to another. Stone rankers also have multiple win effects if they want, they could also have holiday specials.
However, I don’t really know what you mean with kill effects. I know what it is, but could you maybe give an example? There’s a really big difference between some kind of firework animation, or something small, but much more effective. In my opinion firework kill effects are unneseccary. They could harm a lot of players with bad internet, and who knows what might happen next? Maybe they lag out, so if you meant fireworks or something heavy, it’s a no-go for me. Since you didn’t really eleborate on what your opinions are, I assume you could also mean something else. Something that just came to mind, but would be very cool, is that if you kill someone, his playerhead spins above your head for a few seconds, just like the cubelet machine, maybe if not laggy already, with the name next to it. If you have more ideas, feel free to let them know.