I mean to be honest (ignoring the Meta discussion for a second), literally anything from the Monstrous Arcanum book that they didn't get round to would be amazing - I read it recently and the artwork is just

. Obviously I'd love to see the Khemric Titan as much as the next person, but also the Incarnate Elemental of Death looks insane (and redo the incarnate of fire - the art is amazing but I find the model so bleh).
Seriously I'm disappointed in all of you though! this is a Lizardmen forum and NO ONE is going to ask for a new kind of Scaly friend

?!? Even ignoring getting an official version of a thunder lizard (which might be a little disappointing - they just seem to be big bastilodons). We could get some kind of giant Avatar of Sotek that Tehenhauin could ride, maybe as a dual kit with a Couatl. We could get an Arcanodon with some new weapon (since the Stegs stole all their EOTG).
We're lizardmen, a giant brachiosaurus with laser eyes is a thing that would literally be supported by the fluff. We have dinosaurs, we have lasers - put the laser on the tallest dino so no one can hide, infallible logic.
Oh or the Chaos dwarf big fire monster, whatever it's called. That would also be cool i guess.
EDIT: K'daai Destroyer, that's the one (I'd still rather have the brachiosaurus though)