After discussing Season 5 with a friend recently, I've had a few ideas with how I would have handled the ghosts. Instead of having them just be dispersed by water, anything fluid would cause them to loose form. To get around this and actually wreak havoc in Ninjago, they would have to possess things. For example, instead of a ghost piloting the Samurai X mech, it would simply possess the mech as a whole. Some ghosts, in lieu of becoming spooky vehicles or the like, would inhabit their old bodies. Just for a stylistic difference, their bodies would still be partially rotted or covered with lots of armor, to differentiate them from the Skullkin. Morro would be unique, having inhabited a living person instead of an object or dead organic matter.
Having the ghosts be unable to move through air as well as water changes their weakness, as well. Dunking a ghost in water is no problem when it's just free-floating in the air, but the solid matter they would be inhabiting would protect them from such an occurrence. Instead, they would have to get knocked out of their host object, at which point they would disintegrate in the air.
Removing the specific weakness to water would also take away Nya's role as a Deus-Ex-Machina. I'm not saying that she wouldn't still become a water ninja, but the context of it would be different. My thought was that she is almost forced to become a ninja by the loss of her Samurai X equipment. If the ghosts were to raid her little lair almost immediately after entering Ninjago, she would be forced to face them without all of her tech. She's not helpless, but it feels that way to her. At this point, she gets told that she has the potential to be a water ninja. She jumps at the opportunity to learn a new way to kick the ghosts back out of Ninjago.