I think a great idea for an RS update would be a weather system. JaGeX could create an algorithm that gathers data from weather sites and implement it into your chosen region in-game. Here's some effects that weather could have on the game:
- Snow could make it harder to run/walk over ground, and also allow for things like snowball fights or snowman-making.
- Rain could cause armor to rust, with the appropriate Smithing level to fix required, or maybe just talking to a blacksmith or something.
- Fog would make the screen cloudy and misty
- Day and night options according to your timezone would give playing a more realistic feel- darker in the morning and evening, with only torchlight at night to see.
Another idea I have in an in-game "stomach". It could be another little percentage sign by the run energy that shows how full your character is. If you're really full from just eating food or eating food while training, your percentage would be higher and it would harder to run. If it was lower, you'd need to eat food or you'd get disease poisoning from famine, etc.
Let me know what you think; these are only ideas!