After reviewing WotR army list for the Fallen Realms these past few days, my impression is that to stick to a theme would be difficult but personally i would still like to try. In practical terms there are only three theme-able sublists: easterlings, harads, and corsairs as these all have an epic hero each and plastic core troops. Queen Beruthiel was a black numenorean per Tolkien lore but in game terms, could be fielded as any southrons and even as a male character wizard!
Easterling strengths are armored infantry and cavalry which can be supported by light Khandish cavalry and chariots. I don't see much point for Khandish infantry other than as small formation to flank with their 2-handed weapons.
Haradrims can be "upgraded" to serpent guards/riders for +1 Fight and +1 Courage for only 5 points more per company. This seems well worth it to me game wise but the serpent models are all metal, thus financial cost must also be considered. using the taskmaster the haradrims can have +1 per formation to come closer to the serpent troops. The far haradrims of Mahud fluffwise i guess should complement near haradrims but their infantry and cavalry, though great models, do not seems worth the point cost or the money cost to acquire. The half trolls though do seem very interesting. The ultimate back up for the haradrims of course is the Mumak.
The corsairs are light infantry and can have as rare formations the arbalesters as well as the black numenoreans infantry and calvary (Umbar was a black numenorean city after all). I think this gives them a good mix of light infantry, heavy infantry, missile troops and knights.
When taking the ringwraiths, Khamul and the Knight of Umbar seems natural for easterlings and the corsairs as well. The Dark Marshall should go with the black numenoreans (making them common formations) and the corsairs. The remaining 3, the Betrayer, the Undying, and the Shadow Lord i guess are up for grab.