Indeed, Civilization II Test of Time had a mod with air, land, water and underworld planes.

Are you objecting because you worry about lawsuit issues or because you don't like the idea of game magic involving plane shifts or whatnot?

The 4 elementals existing in some sort of altered dimension which can only be accessed by chosen ones; ie: the player. This does present an interesting setting. Each element's influence powerfull in their respective dimension/planes. Scattered around these planes are the respective elemental nodes which prolly yields more magic compared to the normal world.

This could be a serious battleground as players battle to secure more magic and power.

No, I mean that I don't think there would be legal problems because other games like AoW II - SM had mechanics near identical to Myrror so, having not challenged those, the holder of the MoM 2 license could not claim infringement for similar usage in Elemental.

One thing I'd like to see made possible (at least for modding) is a plane with extremely dangerous NPCs, lairs, dungeons and whatnot that the players would be ill-advised to even make contact with until they had really killer armies.  Would be fun to go around busting lairs populated with insanely powerful stuff beyond the ken of the normal world.

Would also be cool if said plane-of-NPCs-of-Doom could send invasions into the "real" world if provoked by the wrong (or right, depending on viewpoint) kind of magic, diplomatic blunder, random event, etc.  And if you get tired of fire elemental armies appearing all over the map, well, deal with it at the source...

Yes, each scenario may or may not have additional planes, and those additional planes might be very different from scenario to scenario.  The question is "what is the range of possibility for a scenario's planes?"

And yea, I'd be a bit surprised if we didn't see the "Elemental" forces of the world take a fairly noticeable role.

It would be interesting to have randomly opening and closing "gates of oblivion" that pop up in different places that offered extremely dangerous opponents with ample reward for taking up the challenge to fight them. It would be neat that if you go through the gate and if it randomly closes while you are on the other side, you will find a way back before you die, which should be an instant game over and that way back my be anywhere on the world when the new gate opens.

Though I really like MoM I did find that the other planes thing always disorentated me. I would move everything on one plane end the turn and boom I had a how 6 cities and large stacks which i had forgoten to move because it was on the other plane, but it wa OK because it was cool, their was only the one and the bonus you gained from owning parts of Myrror were awesome, double magic from nodes, and better loot from dungeons. But when AOW:SM added the shadow plane it really wound me up. So there was three, the underground which rarely contained anything useful, the shadow land which was rubish and all my units got 'shadow blight' (least i think thats what it was called) from being there. And at the start of the game I woud regularly get my city ambushed by some 'sneaky' irritant. And that totaled with the fact I've only ever played it on Vista (I love Vista but it mess up AOW:SM). You got a stupidly over sensitve mouse scrolling, coupled with the fact that my screen has WAY high resolution than the game can support, and its widescreen. Left me strugling for hours trying to figure out what was going on. Eventually i fixed this with a unoffical patch, but by that point I realised A) i wasn't very good at the game(well the producing bigger armies then my rivals part) and B ) the other planes REALLY P***ed me off.

That is how I feel.   besides, you have until 2010 or something to get said 64b OS. all you really need is 32b graphics for the UI to work.

I think an alternate world that exists in the clouds or on flying contentents would be really cool.   You could only access the other clouds by moving back to the other world and moving back in the appropriate spot, using teleporting gateways (i.e. heroes of might and magic 2 portals) or using flying creatures (all the neutral towns there would be flying creatures, most or all flying ships and possibly very low on the build tier.   But would be very expensive for them to create settlers early in the game so that it would be hard to get these races on a different plane)