It should be noted that this is supposed to basically be the "War of the Ancients" or "elf" expansion. Many of the locations on the Broken Isles deal with ancient Night Elven things, which relate directly to characters that are now part of the Alliance. Even then, lots of elven characters (Alliance AND Horde) didn't have major development. Tyrande showed up in Val'sharah and Suramar for a bit. Malfurion helped druids get to their order hall, then stood around in Val'sharah until he was captured, then went into Darkheart Thicket and the Nightmare - and he never showed up again. Vereesa shows up for hunters in their order hall until the Suramar rebellion in 7.1, where she had a "camp" of two high elves and two flags next to the Night Elves, then she was captured in a time lock spell. Shandris had a quest for hunters. Lor'themar didn't appear at all past the Broken Shore, Halduron was a hunter follower, and Rommath appeared for a second in 7.1 as well. Liadrin had, perhaps, some of the most development with WoD and Legion combined, appearing in Talador, Shattrath, Suramar, Light's Hope, DK Campaign, Exodar scenario for pallies, etc. Jarod and Maiev didn't even spend much time doing anything outside of Black Rook Hold.
This was a problem of spreading lore too thin among different classes, characters, and concepts. The setting is much more Alliance-heavy due to elves/elven ruins in Azsuna, Val'sharah, Suramar, the Broken Shore, etc. Considering that most characters that wanted to hunt down the Legion are more Alliance-themed as well, this was somewhat unavoidable. Velen has his moment to shine this expansion, along with Khadgar and the characters listed above. We've always known that the "Army of the Light" was going to be Draenei-centric, because it was brought up through Velen in the first place.
There are a few things for the future that look good for the Horde, however. They have storylines to wrap up for the leadership of the Orcs and Trolls now. If we go to the South Seas, it's entirely possible that we could encounter whatever is left of the "Darkspear Islands" that Vol'jin originally came from. Gallywix got a new, unique model - he could easily make appearances among other Goblins if we go to the Undermine (along with Gazlowe, who is far more Horde-leaning). With the introduction of the Highmountain Tauren, they could have them unite with the Tauren and Taunka (like they were going to in the Legion alpha, before that quest was removed). Sylvanas and the Forsaken have a lot more room to grow, because they are now the leading faction of the Horde, having a pre-established capital of their own.
tldr, the Horde will get their way soon enough. They have more options for improvement than the Alliance at this point, and a lot of Alliance characters have had their spotlight run out in Legion.