Choosing map settings is a fundamental part of every 4(5)X game that I've played. Why shouldn't the mobility of shards be a major setup decision when you start a game?

I've found *most* of the scenarios in this thread interesting, so I'd really prefer the released game to enable me to play one game where shards were like oil wells or uranium mines, another where shards were like crown jewels, and another where they were somewhat in the middle, perhaps like an Egyptian obelisk that a late-Republic general might have hauled back to Rome. Seems like such choices would generate an extremely wide range of play styles, and that any crazy folks managing to master the full range of settings would well deserve an Elemental Hero title of some sort.

How about simply representing the fact that cities can't feed themselves... they rely on the surrounding countryside and imports.  Cut a city off from those and eventually you will starve them out.

I like your comparison!   Altough the "one ring" was certainly worth a seige battle.

I like the idea of fleshing out seiges.  Any enemy army standing adjacent to the outside walls of a walled city should lower or stop food, material, and trade going into the city.  This there seems (by reading some of the descriptions for how materials like metal will be distributed from city to city) a mechanism in place already for a city to store materials and food.  Cities prone to seiges could try to expand these capasity (larger silos?).  It would be cool if cities had the option of building secret project like "secret tunnel out of city" (super project?).  This could negate any penalties to food, materials or trade caused by the seige.  It might also allow armies or heros to secretly leave the castle.  Anyway, I think this would be a fun mega-project to shoot for.  Nothing wrong with a good battle, but seiges do not have to be boring- a bit more thought could make them a real interesting part of the game. 

That seems to assume that cities can only produce one thing at a time, which I doubt will be the case for Elemental. GC2 supports 3-part production, and I'm hoping that the tile-based approach to city building in Elemental means that no two cities need be equal in terms of what they can produce.

Some towns might have a huge guild quarter full of armorers, jewelers, provisioners, and distillers while other towns might have the local castle, a scattering of peasant households with a few inns, and maybe a small mage's tower with a few scholars in residence. So if you can have a 'royal' (super/secret) project sponsored by the castle, plus whatever else your additional production tiles might be able to do...dammit, I'm a really greedy player, aren't I?

From looking at this screenshot I think it looks like production for buildings and units will be separate:

I think the idea would be (with regards to a special tunnelling project) to get it all in place before you actually came under siege.. say if you had a city that looked strategically/geographically likely to be under threat from a siege, or if a city had narrowly escaped it's ruin in the past due to a siege action.

imagine a fortress locked down on all sides by the forces of evil, actually being a bustling metropolis because it has waygates to a bunch of other worlds. 

Well, that is just how Master of Magic does it.   And as far as I know, its the only game that does it, which is why we need a sequal.  Like its part of the Master of Magic hi-concept, so its not my idea...  its the game idea.  It just hasn't been announced yet.  I'm really not sure how anything could be considered a brain-child of master of magic without at least nodding towards it.   I can't take credit for the idea, I just assume its going to be a thing.

That is a good question.  If you have an underground space (or alternate world) map you could counter it just by camping your army outside the town on the other map.  Alternatly, you could just take advantage of the fact they are shoveling resources to supporting a tunnel to over power the town militarily.  Perhaps spells like tremmor or earthquake would automatically demolish the tunnel (you don't fight an earth channeler by digging tunnels.  Like should be on the first page of the rule book)

Threadjacking-I like that term!  Not that I can program my way out of a paper bag, but i think a slider on Shard mobility would be tough for AI.  However it might be able to handle a all or nothing approach (two different behaviors).  Or again, have large unmovealbe shards and also small movable ones (shrads?) that allow some advantage-Maybe three shrads equals a shard.  Anyway, with movable crystals, a good theif, or a magically enhansed hero could preform ledgendary feats (stealing a crystal).  It could shift some power but need not be a game changer.  It would also give reason to fortify your city/fort against thiefs (magical gardians, extra guards, ect).

Fractions without much power could choose to hide the shards in a hidden post or away from the city.  Or maybe in a city.  In this case, the crystal could only be used for defence (perhaps it could be used to make some sort of dome of protection over the area it occupies and adjacent areas).  It would have to be balenced, but this could be used to make a sort of "hidden kingdom" for a limited amount of time for the fraction that owns this (maybe the fog of war is not lifted from this area for enemies and only the strongest of magic users could actually move into this area for say 50 turns).  This hidden kingdom could also have production befifits-creating something like Rivendell or Lorien. The effect would be negated as soon as the crystal is used for anything offensive (help in creating warriors, fireballs, anything).  Once used in offence-the position of the shard would be known to all and all defencive bonuses would be lost.  I also thing this should be limited to one site/fraction.  This would give weaker fractions a way to build strength while preventing the capture of shards from stronger fractions, introduce a neat game option-when to reveal yourself, and again produce a real unique productive city/area that you might hate to see destroyed.  There could be some neat "evil hidden kingdoms" as well.  If this is too hard to code, it might be a nice option for multi-player.