I posted in the MoM thread that one of the coolest and most unique aspects of MoM is the ability to integrate other races into your empire and create cool combined unit armies. I would frequently have High Men Paladins backed up by Halfling Slingers, Elven Longbowmen, Dwarven Steam Cannons, or Dark Elf Warlocks. It's extremely fun.
I believe Elemental has many ways to make use of this. Building a multi-racial army should be difficult, but extremely useful. It will obviously require researching whatever Civilian and Military technologies are required to equip and train special units, but I think it should also require some military and/or diplomatic techs to integrate them into armies without severe penalties. This will have the effect of creating another research path vying for the players research points. That research would be the primary trade-off of going this route as opposed to a singe race route. This is one of the few times I'm thinking that a mechanice from the Heroes of Might and Magic series could be included. In HOMM (at least the last one I played), you had a penalty to morale if you had units from multiple city types, but heroes with morale bonuses could overcome this. I think a similar mechanic would work well in Elemental.
Where I feel Elemental can truly innovate is in how you aquire the ability to build those units. In MoM (and HOMM) you had to conquer a city of that race and it was then rather easy to create those units. This should of course be one viable way to do that, but I think Elemental can also open up diplomatic ways to acheive this. At some point we should be able to make agreements that would either allow us to build cities attractive to a certain race, or to simply have some of them integrate themselves into our existing cities. This should probably be extremely difficult, but be much easier if you have arranged an inter-racial marriage.
I imagine it would be especially interesting if you encounter a race whose unique units (which are probably just equipment and training) have similar research pre-requisites. You would both probably expect the other to want to acquire your race in some way, but the real quesiton will be how. Or perhaps the units would be similar enough that it wouldn't be that big a deal, but I'd rather that not be the case.
One thing I absolutely love about this idea is that it could give "diplomatic" races some real power in the end-game. It would be streching diplomacy beyond the typical idea of interecting with another nation to include interacting with other races and integrating them into your nation, which I think makes sense.