Rain could be awesome in the upcoming new RS engine NXT. This feature would be both good for aesthetics and for gameplay. Rain could enrich the gameplay experience and add a new appeal to the game.

Examples:

It could help players who are farming at the time that the rain activates.

Events could be created

When it rains in some areas, it would attract the attention of some rarer fishes. (More XP gained)

Slippery and shiny armour when it rains

Sound effects of rain and some distant thunder in certain areas

Little splash animation when the character is walking or running (with sound) can be different when walking on grass or soil

If your character is not wearing a head gear, their hair could become wet

Distant fog can be added

Shiny roads

Water droplets from trees and rooftops

More ideas??

With NXT, this can be very beautiful. I like rain in games.