Note: This expands on the ideas explored in "Gameplay Immersion", so it is advised you read that article first.
Think back. Remember when you were a young n00b? How everything was new and exciting? It's not surprising that, by this point, that feeling is a distant memory. Most of you have probably seen most of the game by this point, you've done almost everything you can with your skills, and so on. That's something that is going to happen with every game, after a certain point.
Still, RuneScape has the potential to avert that sensation. Remember Dungeoneering? Players were able to reach level 99, 120, and 200 million experience very quickly, but it's only in the last year that players truly 'mastered' it. The community worked out ideal equipment, monster weaknesses, and technique based on very little information. Now expand that premise to a large land mass and put the player in charge of connecting it to the continent we all know and love.
We've been expecting the Eastern Lands for years now (whether it's actually coming or not), for example. It may seem obvious, but a distant country (or continent) is bound to have different cultures, plants, and animals. For players, this means new monsters to hunt, new ways to train old skills, and new rewards to reap. And if Jagex is interested in preserving that feeling of novelty for would-be explorers, they won't tell us about any of it. In fact, don't even tell us what the resources are. Allow players to bargain with the locals for information: whether they want us to gather a number of items, kill some monsters, or prove ourselves in other ways. Sort of like the slayer skill, but with potion, food, and smithing recipes.
Sound easy? Make the map blank. This is an area that almost nobody from the current RuneScape map has visited, you the player aren't going to know what it's like unless you explore or bargain for information. Even then, there might be resource caches and dungeons that the locals can't or won't tap, which would be perfect for high level players. You might think you see a large empty forest, but it might be a large forest with a level 90 ore rock buried in the middle of it.
It could even incorporate an approval system similar to the one used in Tai-Bwo Wannai. When players arrive, they'll be restricted to the bare minimum resources to allow them to succeed. The more they discover, the more they help the locals through quests or jobs, and/or the more trade flows between regions, the more rewards we'll have access to. This could function similar to Dungeoneering in that you won't be able to take items out of the region, but incorporate the rank system used by the White Knight armor shop in that eventually you will be able to, which could fill the need for an effective money sink.
Different play styles could also be incorporated. Players that focus on noncombat jobs will have access to different resources than players that focus entirely on combat through resource plots similar to farming, so trade between players could be just as important. The locals may also offer cosmetic items and titles to players that differ in the same way.
Unfortunately, such an update probably isn't possible, let alone likely. It's not known if there are any new areas outside of the current, unnamed (to my knowledge) region, and if there is, it will probably be years before we get to see it (if ever). Still, it's something to think about, and to hope for.