I've always been a fan of combination attacks ever since I played Chronotrigger as a kid. Although hasn't this been discussed somewhere before? Well that really doesn't matter I would still love to see some type of magic interaction and combination mechanic for Elemental. 

It would be awesome to be an Air mage and summon a tornado and have your buddy a fire mage cast inferno resulting in a blazing tornado. 

What you mention here sounds like just another good reason to have a Cooperative mode for magic in the game. All the spells will cost mana, and presumably some form of 'attention credits/moves.' MoM let you work on only a single non-combat spell at a time, and that seems reasonable for Elemental also. At least by mid-game, I'd expect any given channeler to have at least a few overland/ritual spells to consider working on. Yes, that Pall of Doom is slowing population growth for everyone but Big Scary Skullface, but am I better off putting up my own Blessing or cooperating with my annoying neighbors to take down the Pall?

I guess what I'm saying is that worrying about co-op magic dispelling other overland spells "immediately" assumes too many things about how easy it is to dispel in the first place, how easy it is to get another channeler's cooperation, and how cross-pressured you might be when it comes to your limited ability to work the Big Stuff.

Also, in the case of 3 channelers all trying to dispel Big Scary Skullface's Pall of Doom, it needn't necessarily be X + X + X = 3X. Another thing is, there would normally be a lot of incentive not to dispel something like Pall of Doom - doing so would require a large investment of time and magic of you for the benefit of nearly everyone else, including (most likely) some of your enemies. Whoever does the dispelling of a global enchantment would essentially be taking a hit for the 'team', where the team includes the people trying to kill you. Cooperative magic could alleviate this issue - but you're right, if not done carefully it could also trivialize some of the big, large-scale stuff.

On a side note, I really hope there'll be a lot of ritual-style spells that require spending several (in extreme cases even very many) turns investing in a spell before it will take effect. And I also hope we'll be able to multitask our channelers' magic use each turn, even when in the midst of a 'ritual'.

I don't even think it would need to be that complicated. Since magic in the game I believe is going to be the equivalent to a rare type of energy you should just be able to cast the same or a complementing spell to achieve the desired results. I like ritual type spells but having to perform a generic one just to combine / strengthen a spell seems kind of lame.

 I think it should largely be situational or based on the physics engine in the game. That way players could end up with all kinds of cool combination spells and effects. Then the only concern would be a bunch of Channelers casting dispel to nullify an extremely powerful spell but that wouldn’t be a problem if you introduced a diminishing return factor in certain situations. For example to nullify the zombie apocalypse spell each Channeler in the group attempting to dispel it would need perform a proportionally more complex spell that would cost a larger amount of mana / essence than normal.