My picks are below.
Each one of these are game changers if done right.
Y’know, I just realized you said Iconic, and not Modern Masters…I’ll be back with a better choice, lol
Edit:
Auriok Champion
Bloodghast
Bogardan Hellkite
Rampaging Baloths
Lightning Helix
These are the two we have already been shown. (I kinda like Magus of the Moon so I would be totally fine if Aether Vial is trashed and the other 9 cards I posted at the top of this thread are created instead.) 
Mana Drain
I actually like the design of Aether Vial okay. Nothing stopping you from letting it fill up a bunch of creatures in your hand and then dumping them all at once. Though the problem with judging spoilered supports is you never know how many shields they start with, which makes a big difference.
Ok, actually… yes. That’s the way to play this Aether Vial. I feel a bit stupid now and I like Aether Vial too now but I still want to see all of those other cards but oh well. I bet we will see a few of the ones I posted though hopefully.
Claustrophobia and Jhesian Thief, for sure 
Though seriously, I think Archangel of Thune would be pretty powerful.
Blizzard Specter, I like options. But with the 3 creature limit the bounce option might be too much. Discarding a card or disabling a creature for a turn is the better fit for PQ I think.
What about them spirit dragons
I think all 5 dragons would be a good set of cards to make masterpieces/elites but they would take up half of the new elite set so I didn’t want to mention them because I want to see more variety.
But yes:
Them Spirit Dragons look good.
They’re awesome but Rivals has a Legendary Elder Dinosour cycle, seems like they would be competing for the same spot.
Oh, we’ll get 10 masterpieces for Ixalan and it’s most likely that we’ll also get 10 for Rivals. So 20 masterpieces in total.
All the Praetors would be awesome: Sheoldred, Jin-Gitaxis, Elesh Norn, Vorinclex, and Urabrask.
I’m thinking that maybe we are getting 10 elites from Masters 25 set (coming out very soon) for Rivals.
I would love love love Sheoldred and Elesh Norn. I think it’s funny that Aether Vial is already a masterpiece in paper KLD block and they are adding it into XLN… but not really. I don’t think it should take up one of the 10 MP slots for XLN no matter how good or bad it ends up being.
First time I pressed insightful and thought “how true… This is the most insightful thing I’ve read all day”! Very true!
None, i don’t think any of them is interesting enough
^Sheoldred and Savageborn and Avacyn especially I’m hoping for out of those. Sheoldred could be the craziest imo.