The only kind of seasonal event I've seen that players like are the kind that gives you easy loot for minimal effort. Cosmetic stuff just aren't enough to impress players unless the ghost costume also lets you shoot lasers from your eyes (and preferably do damage to enemies), and if you give people really cool cosmetic stuff on seasonal events, that makes it hard to have other cool cosemetic stuff elsewhere. If the swimsuit event lets you get your characters totally naked, that will probably be super popular, but then where do you go from there?