Jagex have announced several future updates on the Runescape website. Here is an incomplete list of some of them.
Moderators have announced at least three upcoming quests in Q&As and Developers' Diaries. One that has currently been mentioned by Jagex is Myreque Part 6. Several more Grandmaster quests are also expected, the first of which was While Guthix Sleeps.
There is an underwater area visible from the Saradomin Encampment in the God Wars Dungeon. It was discovered on 8 November 2009. It contains a skeleton on an altar, as well as some bubbles. Also, a new land called Acheron is expected, containing "penguins, mammoths, polar bears and more".
Multiple rumors have been going on supporting the fact that in Summer 2010, Jagex will reveal Green & Black Dragons to F2P servers. Another rumor states that a new demon will be added to the Greater & Lesser line, the Minima Demon, at level 52. These rumors have not been confirmed and should not be trusted yet.
A full set of equipable items gained from Achievement Diaries is an expected upcoming update. Other upcoming items are the Dragon Crossbow, the Dragon Warhammer and Dragon Metal used for smithing.
Impetuous Impulses will receive a large update. Also expected are a rebalancing of the Combat Triangle and a new high-level dungeon.