Introducing more variety
First off, this is all conjecture - I don't know what the story of Warlords of Draenor is going to be. We'll be ranging across a large and varied continent, with hundreds of new quests telling the story. It's possible all of this has already been written out, but let us run with the idea anyway. What good are the smaller clans to the narrative of the Iron Horde?
For starters, they give us a chance to portray orcs with more subtlety and depth. After Mists of Pandaria, orcs have kind of an image problem - the vast majority of Garrosh Hellscream's loyal forces were orcs, after all, and the Horde rebels were primarily tauren, trolls and the other factions. As a result, even if there were orc players opposing Garrosh it was hard to see orc feet on the ground fighting him, and thus, orcs appear to be a singular group with figures like Zaela, Nazgrim and Malkorok supporting the Warchief. There's not a lot of establish orcish dissent - Thrall, Eitrigg and Saurfang opposed Garrosh, yes, but their roles were small in the overall storyline.
It's not a good idea to repeat that mistake. Yes, Horde players will be opposing the Iron Horde. But there need to be those orcs native to Draenor who oppose it as well, whether out of simple insularity, a desire to maintain their established ways of life, or a flat out disdain for or disagreement with the Iron Horde and its goals. Clans like the Bladewind stood up to Ner'zhul and later Gul'dan when they were working to forge the Old Horde at Kil'jaeden's behest - why should they follow the Iron Horde? The rise of the Iron Horde seems to be fairly swift, but if it is to be credible there has to be some opposition when we get there.
How the game can better represent its players
There need to be more women characters because a lot of women play World of Warcraft, and the game needs to reflect them. They're a big part of why it's as popular and widespread as it is. The Mists of Pandaria approach to women, simply having them without calling much attention to it, seems to me to be the one to take. The diversity of orcish clan life is tailor made for this approach. You can't easily replace figures like Ner'zhul or Grom as the head of their respective clans - we know who should be there. But in addition to having prominent women in these clans (more figures like Draka of the Frostwolves for all clans) clans like the Thunderlord present a golden opportunity to show orc women in positions of authority. Fenris and Garm Wolfbrother were defiant and willful leaders. It's very plausible that Garrosh may have been forced to remove them and replaced them with someone more ambitious - he's done it before. A sister or cousin could easily rise to power, especially since Garrosh seems far less concerned with gender than the older generation of orcs he's trying to work with to forge this Iron Horde.
Similarly, clans like the Rageroar and Dark Scar were invented for other games and either entirely wiped out before those games started or never seen before or since - they're literally blank slates to be shaped into whatever would suit the story of Warlords. A series of quests about the Dark Scar clan and the origins of Serathil could easily lead to the establishing of a new clan leader, and there's no reason it can't be a woman. Seeing more women in the ranks of the Iron Horde and more women in clans opposed to the Iron Horde makes sense, because the Iron Horde itself is a huge change and a massive shake-up to the traditional orcish way of life - some women would of course want to take advantage of this, and others would want to maintain the life they've always known.
To use an example from the lore, Griselda Blackhand is a character who was ambitious and constrained by her family and way of life - aged to adulthood by demonic sorcery and trained to fight, then denied the chance to drink the Blood of Mannoroth or do any actual fighting, she rebelled against her father and joined a band of renegade ogres led by Turok. Her father had her killed for this. What happens to Griselda in the new order of things? She doubtlessly ages normally - does her father prevent her from being trained, this time? Does she still rebel? This is a character who really didn't get the development she needed last time - now, everything could change for her. And she's not the only orcish woman who might see in the Iron Horde a new path for her traditionalist, custom-bound people.
In the end, these interests dovetail. To show the orcs as more than just a one-dimensional army of conquest (that element should be present, surely) we need to see some of the clans we haven't yet, and we need to see more women doing things - fighting for the Iron Horde, fighting against it, seeking to avoid it, profit from it, or dealing with the consequences of it.