Gilgoblins are an experiment gone wrong, and were created by Hobart Grapplehammer (implied while under the effects of Kaja'Cola; "goblins with gills" is one of the random ideas it gives you after consumption). They were given various aquatic features which allow them to survive in a wide variety of aquatic habitats. While seemingly not as clever as their greener cousins, they aren't really dumb as people say. They're capable of speaking coherently, and Gilblingle at least seems eloquent enough in writing:
"Our self-proclaimed king will stop at nothing to acquire my family fortune. He has threatened and harassed me to no end. I can't take it anymore! I've buried the treasure and fed the map to Blackfin. That should keep my cousin's grubby little mitts off our precious heirlooms!"
Gilgoblins are not stupid, just wilder and not as smart as other goblins. Another misconception is that they cannot survive out of water. They can clearly breathe air and you can see many hanging out on dry land, and there's no reason to think that they would dry out. They're goblins with fish features, not actual fish.
Very likely classes:
Mage: Descend from a mage race, and have extensive contact with naga arcane users. Gilgoblin "aquamages" exist, which are presumably related.
Hunter: No reason why they can't tame any undersea beasts. Their starter pet could be a crab like the goblins, but of the hermit or spined variety. Nazjatar is also introducing some swimmy critters, like nudibranchs and snap dragons.
Shaman: Descend from a shamanistic race, and have a connection to the element of water.
Rogue: They have hands and no hooves, and a rogue even has its own card in Hearthstone.
Warrior: They have hands.
Likely:
Priest: Probably have some connection to Nzoth, and have contact with followers of the void.
Possible:
Monk: They have the fighting style down, but lack the mentality.
Warlock: Doesn't seem to be a thing for them, although they could pick it up.
Druid has also been thrown around a lot, and may have some traction. All three of our gilgoblin companions in 8.2 have Gral's blessing. This means they may have a connection to an ocean loa, and therefore to the Emerald Dream.
Miscellaneous Possible Racials:
Melee: Biting with shark-like teeth.
Snare: Throwing a net or spear.
Water ranged attack: They have water magic.
Underwater breathing: GILLgoblins. To avoid stealing another racial, it could be a castable oxygen bar replenisher.
Increased swim speed: They're aquatic after all
Increased fishing skill: What else is there to eat in Vashjir
Increased loot percentage: They're plunderers and pirates
Reduced snare duration: Slippery and wet skin.
Possible Secondary Languages:
Nazja: Taught to them by the naga.
Goblin: Their original language.
Nerglish: Picked up from murlocs and makrura.
Their own dialect: ???
Heritage Armor: Either something makeshift like sea giant armor, which is made up of things like nets, starfish, and seaweed, or something more naga themed with enchanted metallic armor and globes of water. In the former case with organic armor, maybe it could "bloom" into polyps and anemones while you're underwater.
Tattoos: They already sport inky tattoos that take the shapes of swirls and waves.
So far the skin colors seen on Npcs are: Dark blue, light blue, green blue, and red. Eye colors to tie into faces would be: Gold, bright blue, orange, and dark blue.
Some other customization can include different kinds of fins, gills, and makeshift jewelry. Even some coverings like barnacles or weeds.
I'm terrible at flirts and jokes, but tried to fit some references in with the cheap puns. They'd be split between male and female as seen fit.
Flirts: "Do you find me a-lure-ing?" (Pun and wc3 naga quote) "I'm hooked on you"(Pun) "There's plenty of people on land" (Plenty of fish in the sea) "We should hook up" (Pun) "Aren't you a rare catch" (Pun) "Let me tide you over" (Pun) "Let's sleep with the fishes tonight" (You'll be sleeping with the fishes) "Two parts heart, one part obsession" (H 2 O) "I'll take your breath away" (Pun) "Fish chase anything that glitters." (Moana reference) "Are you a natural blue?" (Nemo reference) "I ship it" (Pun) "I'll dock in your port any day" (Not really a pun) Also something in regards to "booty bay" that isn't too terribly obvious :p
Jokes: "Tide is money friend" ("Time is money friend") "He lives and reigns and conquers the pool" (Thessalonike reference) "I keep my goldfish in the river bank" (Pun) "No I'm not a shellfish" (Character misheard "selfish") "Loan shark? I'm actually rather chummy" (Punz) "You'll swim too" (IT reference) "I'm thinking of something orange, and it's small" (Nemo reference) "My leg!" (Do I even have to say it) I disdain all glittering-ooh, shiny!" (Pirates of the Caribbean reference) "Something's fishy.." (Pun)
Recruitment Scenarios: There are various ways we could recruit them into the Horde.
1: We make more: Sylvanas could pressure Hobart into making aquatic supersoldiers for her naval fleets. Shenanigans ensue, and Greely gets to be a guinea pig.
2: They prove their worth: We proceed in an effort to destroy the faulty creations once and for all, only to have second thoughts.
3: We befriend a tribe: Gilgoblins don't always serve naga, many are independent. We could happen across an outpost of friendlier gilgoblins, and earn rep by fishing or buying from them. Edit: And we now have a gilgoblin faction! https://us.forums.blizzard.com/en/wow/t/gilblins-as-new-allies-the-unshackled/148710
Also check out these other allied race threads that'll die if you don't have mercy on their tinfoil hats ;-;
Darkfallen: https://us.forums.blizzard.com/en/wow/t/sanlayn-darkfallen-official-megathread/18803
Arakkoa/Ogre: https://us.forums.blizzard.com/en/wow/t/arakkoa-ogre-megathread-pt-2/10359
Sethrak: https://us.forums.blizzard.com/en/wow/t/unofficial-sethrak-discussion-thread/18815
Vulpera: https://us.forums.blizzard.com/en/wow/t/vulpera-discussion-megathread/18751
Junker Gnomes: https://us.forums.blizzard.com/en/wow/t/mechagnome-allied-race-discussion/147354