Well, Cities does have fantastical creatures (Dragons and Griffins anyone?)
Their focus is still less on the fantastical. Their generals get a fancy mount, but virtually everyone is a relativly "normal" mortal. Even those mounts are relativly mundane compared to others. There's less magical abilities, less artifacts, less weird stuff like our faction which has dinosaurs riding on top of other dinosaurs while operating a laser. It remains a fantasy faction so of course there's the occasional wizard or fancy mount, but it's the most mundane of all factions.
and to be honest the Empire models just don’t fit with the new Sigmarite aesthetic because they’ve got the old motifs of comets and skulls everywhere rather than the new hammers and lightning bolts that the Sigmarines have. I think GW should just scrap Empire and replace it with a new human faction, and work on fitting Dispossessed, Dark Elves, High Elves and Wood Elves into AoS somehow.
Honestly, they could just allow the old models to remain legal, and even sell them while they stay profitable. Just start introducing more modern sculpts of the same stuff. They're supposed to be a mismatched militia of various cities anyway. So not having a uniform look fits perfectly fine. Just say this particular knight is fighting in old heirloom armour, while the next has more modern armour. Doesn't need to be a fully new faction, just slowly phase them out of production while allowing them to stay game legal. Old players can use their collections, it fits with the fluff, and the faction gets a much needed makeover.
Perhaps they are actually making a new human faction and the upcoming Vampire Hunter/Witch Hunter model may have something to do with it?
Honestly, I'd doubt it. They haven't made a single new "normal" mortal since AoS started. I'd be very surprised if another faction pops up.
Plus, they've kinda written themselves into a corner, since every modern human city basicly belongs to CoS according to the fluff. All the old ones were either razed or corrupted by chaos. And it's not like the idoneth where you could say they're hiding in the sea, or the Lumineth who have an entire realm to their own. It'd be rather difficult to fit in and explain why we haven't seen them yet.
Speak for yourself, I’ve had plenty of ideas that I’ve mentioned before in various threads, and I think they’re great. In fact I’m going to write unofficial warscrolls for them until GW see them and copy the ideas, because I can do that much to further the cause of one of my favourite factions.
I am speaking for myself

And I know, most of the ideas I've seen you mention just don't really do anything for me. They're not even terrible ideas on their own. But the fyreslayers are just so terribly one-note that most of the new stuff is either just more of the same stuff (which isn't interesting), or seems to be a massive departure from the established core (which is jarring). Neither of which are exactly things I would get excited for
Now, how GW would actually go about this is uncertain (they could have used Dispossessed for this if it wasn’t for their stupid, stupid, STUPID idea of merging them into Cities of Sigmar with Empire and Dark Elves), but there is premise there.
What's the promise though? Aside from "this is their goddes".
What are their cultural values, warstyle etc. and most importantly, how does that significantly differ from the fyreslayers, dispossed & KO?