It "could" be cannon if the devs wanted it to be. We really haven't seen much of the back-story yet. I think Flying Citadels would come in very handy. It's the ultimate Fantasy mobile troop center. A Flying Base. It would be like the Ultimate planet destroying ships you get in Gal Civ 2, Doom Stars I think they're called. End Game units of course. Not cheap and not easy to build with an insanely high cost and upkeep. 

That would transform the city into a unit ? It reminds me of the fatboy or Czar in Supreme Commander.

The easiest way to implement this would be to make the 'flying' upgrade simply a super version of a moat.  Prestige goes up, defense goes up, but other than that nothing changes.  The city can't move, even though visually it looks like it's floating.

This got me thinking. Will we be able to have cities underground, inside mountains, *coughMoriacough* etc? Underground cities would give a big bonus to defense and mining, but increases upkeep because food has to come from somewhere.

*coughThorbardincough*

Everyone always goes for the LotR reference first.

Don't get me wrong, I like LotR as much as the next guy. Tolkien was a god among writers but he's not all there is in Fantasy. There are many other great books out there that don't get the credit they deserve IMO. If they would have made Dragonlance into a live action movie before LotR people would be saying "Lord of the what?". Honestly with all the war and dragons I think Dragonlance would have made a much better movie if done with the same high standards and quality LotR was.

I went with the LotR reference because a) Pretty much everyone has seen and/or read LotR, so they will get the reference, and I'm going to be making the LotR mod for Elemental, and having underground cities already in the game would make it easier than actually modding them in

Indeed it is. I'll be looking for the same thing for my Dragonlance Mod

Those are some very good points. That would all depend on what the Devs would want to do with it. If it becomes a unit and stops being a city then it won't have a area of influence. I also don't see the need for it to go too slow but I figure if they would put anything that powerful in the game it would have to have some down sides as well. Balance could be an issue with what-ever other things they already have planned. In my mind a Citadel should move. I can see a variant of the spell being used to add a bonus and keep it stationary as Strand suggested. Like a floating version of Wall of Fire or Cold etc etc. A city protection buff.

I've wanted the same thing. One of the best draws to the original MoM is that every time you play a game you get two whole worlds to conquer. I don't know if Elemental is going to have other environments but I've heard them talk about dungeons so I'm guessing we'll actually be able to go inside them. I'm hoping at least..lol.

Different planes would be a very good idea. But as everything, only if well done. One mod I have in mind would need 5 different planes to be considered complete (can survive with just 1 tough). But Frogboy already said something about land based TBS so doubtfull unless very good arguments against that are given (and money and time allow, of course).

And aerial plane could be a nice addition for flying things but I'm not sure it's really needed. In the 3d engine you would see them lying normally in the map and in the cloth map, it would be just a figurine lying of the surface of the map.

Undersea map it's more interesting and problematic. In the cloth map the figurine would behave like any other (over the cloth map) but in 3d game it would depend. A sunken city would be underwater and you should be able to notice it without the need of cloth map. A giant turtle could be in the surface but could interact with the sunken city if in range.

Different worlds (underworlds included) would be nice if you already have an interesting world and if you can make the new ones interesting too.

But the topic was about flying cities (I prefer sunken ones) so let the flying citadel detroy the area where it was before (at least forsaken or something). The cost could be mantained but it would be cool if the cost could be paid once and the effects sustained by itself (once magic skill is good enough). I can see people using "It's raining flying citadels" tactics if the game allows them. Maybe to make it fly it's a one time payment but o make it move requires extra effort (be it essence and/or mana, preferably the later).

I would say that an underground would add the dimension of knowing the lay of the underworld, your could put stronger monsters down there, of course it would not be as large as the map above, just tunnel's and such that lead to different area's of the map, or deeper into the world where even more devious creatures live. better treasures etc. I have not seen dungeon's yet this may be what they have in mind, it would be a completely different enviroment from the way the main map looks, and as for a floating area of the world, it wouldn't be as big as the regular make, maybe a few interconnected sky island's that flying creatures settled on, just something to add a little more exploring to the game, I like those moments where you find something secret, and are excited that you have, just like in Heroes of might and magic when you found the underground area you always thought it was something cool. but it turned out not to be because of their lack of imagination in underground enviroments, of course at this point anything is possible.

If its a flying citadel, it should most likely either have

A) a magical, gigantic beam cannon

or

constant magical storm overhead, flooding or burning most all it passes over. (massive rain and lightning, perhaps fireballs/meteors)