Since the idea is to base this off of the D20 system, not having one of the big 4 classes (by that I mean Fighter, Cleric, Wizard, Rogue... where it all began) seems to be a big omission.
So if we want to fix that then maybe there should be a "Thieves Guild" building that enables you to make Rogues.

Some idea of what a rogue might do.
Hide/Sneak - Rogue units do not display on the world map to non-allied factions unless adjacent to a unit of that faction. (this applies to the rogue unit only and NOT other units when stacked)
This means rogues can scout around in enemy territory without the faction knowing, without breaking treaties, etc. unless an enemy moves by them or they move by it.
Sneak attack - Rogue units get extra damage in combat when flanking, attacking paralyzed units, etc.
Balance - less likely to fall prone in tactical combat on slick surfaces (ice)
Tumble - reduced fall damage by 10' (DC 15 - rogue skill assumed to be 3 + level + dex bonus - armor check penalty)
move at half movement without provoking an attack of opportunity. (DC 15)
move "through" an enemy unit without provoking an attack of opportunity (DC 25)

Rogue heroes might also be able to get loot from SOME ruins, use a planar gate, etc. without having to fight the guardian.

Not saying all these would need to be in, heck Rogues don't need to be in at all, just thought I'd brainstorm how they could be added and provide some use since traps and secret doors aren't really present.