So if we round up the rumours around the Idoneth Deepkin:
Follow Mathlann (who we know from WHFB was a bit of a "difficult" God, represented Poseidon in the game)... which I know was more High Elf before but the High Elves themselves didn't like Mathlann that much. If Mathlann is more like Khaine...
A couple of heroes - one riding some sort of sea creature, or otherwise (magic?) - I personally don't think Mistweaver will be part of them despite...
... Them coming from the mists and magically affect the areas they raid by making it as though it's "underwater"
Greek aesthetic (ties in with Atlantis, Poseidon) - helms, spears (makes sense) and by extension harpoons?
Shark cavalry or chariots?
Giant turtle, ridden by Aelves? Is this a behemoth, a chariot, a war machine? If not behemoth, could there be more to come?
Soul-stealing - does this relate to Teclis failing to create a race of perfect new Aelves, who just somehow aren't magically bound the souls recovered from Slaanesh? "Woe is us, lets retreat to sea and come out to take what we can get". Does it link to Nagash? Hide in the deep to disguise their tenuous hold on their Aelven souls? This I guess also pitches them again Chaos (would be anyway) and against destruction.
8 lines - 8 units from combined boxes (very common now, and a good idea) seems likely.
Kraken/octopus logo.
They're Order (get over it )
They're not "High Aelves" (get over it )
My feelings:
Inspired by Atlantis, Greek Gods. Spears, harpoons, tridents, helmets. Fish scale style-armour. Could even be a little bit African even, if you look at some of the Atlantis stories. All with an AoS twist.
Would have been High Aelves, but by virtue of stealing souls they just can't be. Doesn't make them wholly evil, chaotic, death or destruction (hence Order), just like the Dark Aelves themselves really (bar Morathi, but she hates Chaos and Nagash so that's that). Bitter?
Not all of them will need to steal souls. Some perhaps are chosen of Mathlann, hence lead their kin to land.
Don't think we'll see fish Aelves, rather more souless, more undead looking. Gaunt. Sad. Angry? (I can't help but think of Little Mermaid and those stolen souls the fat Octopus woman took ()
I think rumours will emerge of their battleline units. Nothing we've heard so far screams battleline to me yet. This has got to be a dual kit like the Melusai and Khinerai? I think they'll be the ones above.
I think rumours will emerge of a larger sea creature than an Aelf riding a nasty turtle, though honestly don't know. Snapping turtles are mean though!
Some mechanic like the Sylvaneth woods. Hope it's not that bloody boat though! Stargate-esque Portals? Water funnels? Something cool.
Can't see it being a horde army