Approximately a third of Istaria is wasted space as it contains only the oceans. A relatively easy way to expand Horizons without increasing a whole lot of server load would be to deepen the oceans and add underwater cities. Of course, waterbreathing would have to be introduced (Possibly making the Helian Dragons able to breathe underwater as the Lunus overpower them with melee already) Swimming for bipeds could be treated similar to flying for dragons as far as the controls and mechanisms go. Creatures underwater would add a whole new dimension to the game also.
Hate to hop onto the whole "But the other MMORPGS..." bandwagon, but it is true that most other MMORPGs have underwater creatures and combat.
Just a thought.