I really like the swimming Skimmer and the Legion Waystations. Let's post some mastery ideas of our own that we'd like to see.
Mount tier masteries:
1) Raptor and Jackal: Faster endurance regeneration. This would allow them to use their speed boosts more often.
2) Springer: Another tier that lets springers jump higher and/or faster. The skyscale almost completely negates the springer right now.
3) Roller Beetle: A mastery that let's them keep their momentum when going uphill.
4) A mastery with a long cooldown that let's players mount in combat.
5) A mastery that let's players switch mounts instantly. Right now, if a player on a raptor wants to switch to the springer, the player has to dismount the raptor, land on the ground, then mount the springer. With instant mount switching, if a player on a raptor switches to the springer, the player would jump off the raptor and land on the springer, without the need to dismount and land.
1) A mastery that automatically opens treasure/meta chests if the player stands still on a chest (the player would still need corresponding keys). This would greatly help to deal with trolls that place items and portals on chests. It can be further upgraded for AOE chest looting. Disabling auto loot in the options will disable this mastery.
Special action masteries (similar to the Legion Waystation):
1) A mastery with a long cooldown that launches the player like 50 yards into the air (see Aviana's Feather in WoW). Great when combined with the griffon if you need to get somewhere in a hurry, like before an event that you desperately need ends.
2) A spiritual bird that the player can control to fly around the map at very high speed (as fast as a boosted roller beetle or griffon) and scout the map. Not the most useful sounding, I know, but it would be useful for something like daily map event completer. The player can use the bird to fly around at high speeds and see if any events are up. Once a player finds an event, they can teleport to the closest WP and rush there. Plus, it would be just fun to use and Anet can sell skins for the bird.