Alright, maybe it's not very original nowadays, but for a strategy game, I think it's pretty unique. I searched, but I didn't find anything like this (but the Campaign of Tactical Battles is sort of the same idea).

I'll just state that I have no modding experience whatsoever, and I have exactly 0 mods under my belt. This is just an idea for some modders to pick up and run with.

INTRO:

So, for anyone who doesn't know what Horde mode is, it's a Gears of War 2 game mode (and a similar mode is in Halo ODST and other games) where a few players are against increasingly difficult "hordes" of enemies. After one wave of enemies is finished, you have a minute or so before the next wave comes, and there's more enemies and they're stronger.

BASIC IDEAS/POINTS:

1. Start with a random map, or a set map;

2. Choose type of heroes/units to be with your main unit, a wizard I'm guessing (or maybe no units will help you, maybe it can be a "1 against many" situation, or a few players' characters against many hordes), and they start with basic equipment (or not); choose difficulty level/number of waves of hordes, number of players, etc (most horde type games are multiplayer, so if no human players are available, then maybe have a computer on your side?);

3. Small waves of easy enemies will attack, then maybe a pause between waves of hordes for resting, etc. ("the next wave will attack in 5 turns");

4. Since Elemental is almost an rpg, there could be a lot of options next: gaining experience to more easily survive the next wave; new skills, maybe gain "points" for kills to buy new equipment; level up to gain spells for your hero/wizard; maybe use these points to recruit new units to help you;

5. Maybe have a city/fortress to protect (as opposed to just having the enemies attacking you; in this way, it's almost like a tower building game; the fortress has hit points, and when enemies get through and you can't stop them, it takes some damage; maybe an enemy gets one free hit on your fortress, and after, say, 50 hit points worth of damage, you lose (of course this number should go up a bit for each successive wave you pass);

6. Because this is not real time (is this even possible, HMM, ideas?), there could be/ would have to be more strategy involved in the fighting. It shouldn't be just about getting into the tactical battles (which should be a big part of the mod, of course) but you should also be able to use spells to affect the map and cause damage from outside tactical combat. If 4 stacks of enemies are approaching, you should have some spells in your book that will maybe stun/stasis some of them to make it easier, or affect the landscape to force them to come at you differently (just brainstorming, there's probably tons of things that you could do, and I bet some of the spells in the real game would fit just as well). And because there's a limited number of units under control of the players, there should be adequate healing, resting options to keep the game enjoyable; otherwise it'll be too hard;

7. Another combat situation might be what was discussed in another mod idea: having the combat take place in the world map; this way, the other players on your side could join in large battles, instead of just, you fighting, then your friend does a fight, etc. Perhaps in the harder waves of a multiplayer game (20+ waves?) the enemies will start ganging up on you, becoming bigger and bigger stacks of units; to survive these maybe you have to team up with the other players in the battles (i.e. 3 people's wizards in the same fight against a ton of enemies). Ideas?

That's all I can think of right now. It's super late, but I just wanted to get this idea out there. I hope some of you think the idea is worthwhile. Maybe some people with modding skill (not me, I'm just supplying the idea) can respond and shoot some ideas back and forth.