I would love to see a mod where you pick a leader at launch and it would determine the style of the army you get when starting
out, champions and lesser units.
At the beginning, most of your buildings would already be built, youd get most of your units and most of the game would be fights. A good mod for people who enjoy tactical mode in multiplayer. Special attention would be given to make great tactical maps (with choke points, elevation, +def -def. Theres a tactical map editor coming... right?) so youd get that final fantasy tactics feel to the tactical maps, where they are often quite different from one another:

You would get a nice roster of leaders to pick from, such as this hero list from Heroes of Newerth:
http://www.heroesofnewerth.com/heroes.php

So for example, if your Lord is Predator, the one framed in Yellow, you would get an army that is wolf and werewolf themed.
If your Lord is Devourer, the one framed in red, you would get undead units, skeleton armies and ghosts, etc...
If your Lord is Arachna, the one framed in blue, you would get spider themed units, web spells, poison spells etc, etc...
If your Lord is scout, then stealth related spells, bows, etc...
Again, most of the game would be spent leveling up different Hero units, your lord, and probably hunting items around the map and trying to acquire unique units. Could have some 2vs2 maps, 4vs4, 8vs8, FFA.
Its late and im tired, if you like this idea you can add some input of your own to see where we could take it.