It has been alluded to, several times in fact, throughout the course of the game. From the initial "tutorial island" to the latest Elsweyr expansion including its pirates and "swashbuckler" personality - it certainly feels like ESO is ripe for an expansion that fully embraces the nautical theme. And I, for one, would absolutely LOVE it!

Your "mount" would be a ship. Customization options and crates available to make it your own. No need to get too challenging with it, but while on the ship your skills bar is tailored to ship combat and a whole new line of skills become available that you can use to pirate, defend, or simply sail around chasing down mobs. A group of ship could sail together, essentially becoming a flotilla, and take on bigger challenges. Hell, this would open up entire new "dungeons" that happen in the open seas where you, a group, or your guild could work together to take on tougher challenges. Instead of way-points being used as a means to respawn should your ship be taken down, various ports would become available around the continent where you can respawn, transition from sea to land, or sell/barter for goods.

Tank Build - The Man O War - Large, slow, but devastating in battle. One broadside was enough to nearly end your naval career and it's now turning its other side to face you. Pray to the divines for a sudden storm or a strong tailwind because if this hits you you will be meeting them soon anyway.

Healer / Support Build - The Brigantine - Light, fast, and easy to maneuver. You may not have a lot of firepower, but you specialize in moving in and out of the fray while turning the tide of battle with well placed and highly effective mage mines - of your chosen element of course.

DPS - The Frigate - Not as large as the Man o War, not as small as the Brigantine, you're somewhere in the middle and specialize in using your more sleek build to swiftly move into the fray at critical moments to seek out the enemy ships weak spots. These spots can be highlighted for other ships in the area to focus in on, or, you can use a mixture of targeted firepower and armed militia to either make those areas larger and more likely to be hit by others as a result - or gradually weaken the other ships crew and their effective firepower as a result.

I know this is likely just a pipe dream - but it was a hell of a lot of fun just to write and have fun with.