Well, yeah, not saying the playable-type Forsaken should go away, just that I think having only them and few types of ghouls of abominations is not optimal. It puts some pretty nice models in the void, it is a missed opportunity for the story development of the Forsaken (in my opinion, I think interactions of various types of free undead could lead to some interesting things) as is a missed opportunity for the Forsaken themselves in-story, because hey, free manpower. It also goes against what Sylvanas said in 3.3 after Arthas died.
For example, my main focus in regards to this idea has always been the San'layn since I consider them to make the perfect fit for the Forsaken. Considering their extremely high ranking position in the Scourge as well as ties to Sylvanas extending to when both them and Sylvanas have been alive, they could be arrogant bastards and try to put themselves high in Forsaken hierarchy, leading to internal development for not just the San'layn, but also the normal type of the Forsaken standing up for themselves. The San'layn could have also tried gaining the support of the Dark Rangers and Banshees which are pretty much in the same boat.
As for the normal Forsaken, Forsaken have tons of different sub-groups that are completely underutilized. Are Dreadguards assholes to Deathguards because they are the elite force, akin to what I just said would be a possible outcome of San'layn joining? What about Executors, considering their position bleeds into civil sector as well in some cases (which could also lead to fleshing out that particular position). What are the relations between Shadowstalkers and Deathstalkers? What is the position towards both of them by other groups (especially Dreadguards)? Are they seen as compromised due to being under Varimathras? What about the Death Knights that have joined them? We know Koltira wasn't the only one. Did they know about Koltira's fate? Did they leave because of it or saw it acceptable given his treason? Then there's RAS, which has opportunity for both political maneuvering as well as their usual research stuff and redeeming themselves for Putress (well, in the eyes of the Forsaken and maybe the Horde, not going full "by the Light"). And maybe some development in relation to subgroups within RAS, like clarifying what the position of Plaguebringers is.
And finally, the more task oriented groups that consist of members of all of the above, i.e. Hand of Vengeance and Defilers. Do they have special priviledges? What are Defilers up to now that Galen has betrayed the Forsaken, then ruled Stormguarde, then led to its complete destruction? Is Hand of Vengeance still a thing? I recall that Warsong Hold has been mostly abandoned, but Forsaken, given Sylvanas' interest in freed undead expressed in 3.3 as well as her learning of the new Lich King and not being all too happy about it could have led to them maintaining forces there (and well, in light of Wrathgate having bases in frozen shithole the rest of the Horde is no longer interested in could be beneficial). Given how Bolvar is finally becoming more active, Hand of Vengeance still being a thing could also lead to interactions with the Lich King, Scourge and even Ebon Blade. And maybe even to Ebon Blade realizing Bolvar has been playing them and that at least causing a split in the group and that split subgroup joining the Forsaken en masse (another thing I'd like).
At least some of these groups have leaders which are already established as high ranking Forsaken (and the rest logically should as well, we just don't know the leaders, Dreadguards being an example here). Black Bride, Grand Executor Mortuus, High Executioner Anselm, Lady Cozwynn, Aleric Hawkins (apparently was active during the Legion invasions in pre-patch), Master Apothecary Faranell, Doctor Sintar Malefious (the only other known Grand Apothecary and the only remaining one after Putress' betrayal), Apothecary Lydon (he may lack in rank within RAS, but seems to have a penchant for politics). All of them (and more, vide the yet unkown leaders of some of these groups) should be much more important characters and share some of the spotlight currently clogged almost entirely by Sylvanas (and now Nathanos a bit, which, while an upgrade is also emphasizing the problem in a way). And by that they would flesh out their respective groups (well, support characters would also be welcome). These two would in turn amount for a lot of development for the entirety of the Forsaken.
And then there's the huge potential of old Forsaken vs new Forsaken, also centered about playable type. How do the new ones fit in? How do the old ones treat them? Are there sentiments to limit the advancement of the new Forsaken among the old ones? Do these two groups view the undead risen by Val'kyr that decided not to join the Forsaken differently? So on and so forth. Speaking of those who did not join, they also a potential hook, after all Rotbrains shouldn't be the only ones to do so.
Some could just explore the world and try to find their own place in the world like Forsaken did in the past and giving crazy quests for both sides. Some could maybe try to settle in Lordaeron and establish their own settlements, maybe a small state even and attempt maintain neutral-to-positive relations with Forsaken, just not wanting to be under their rule. Which would then lead to how Forsaken would treat that. Would they view it as OK, because they can understand each other better than the living? Would they accept it in the vein of a buffer zone against Alliance invasion? Would they turn hostile?
Back to these undead but staying on hostility, some of course could be hostile and we could see another group like Rotbrain appear, with their own leaders and all. And since the Alliance has been mentioned, what if the hostile ones tried to gain their support? Which in turn could translate to more fleshed out and in-depth story for the other faction as well. Would the Alliance accept that? Anduin could be open to the idea (though on the other hand he would also be pro-peace so maybe he'd give the condition of not seeking further conflict)? Would Genn try to block it? Or would he see it as ironic karma for Sylvanas and welcome them with open paws?
And with all of these normal looking Forsaken (that is if Blizzard was actually competent at developing more than 1-2 characters for races that aren't humans and Orcs) there would still be space for other types of undead within their kingdom. Few Liches (Helcular, Arcanus and maybe some female Lich to not let Deathwhisper's model rot in the void becoming Archliches of the Forsaken with some lower ranking ones here and there), remaining San'layn, developing Dark Rangers and Banshees further, maybe fleshing out the Ghouls, Abominations and the like (while also making them more varied), and potentially even one Marrowgar-like undead and few undead Nerubians that couldn't find their place in Azjol-Nerub.
Hell, not to mention the ghosts of Alliance Expedition that joined them all the way back in TBC. And apparently there is even a skeletal Orc since WoD (completely missed it), which is theorized to have been resurrected during Siege of Orgrimmar, which opens some more doors. One, Val'kyr growing even more stronger, which in itself is a story hook, but also, if that's the case, how do Orcs view it? Especially now that Sylvanas is Warchief. Oh, and of course the Val'kyr themselves. They are pretty important for the Forsaken society and yet we don't even know the names of the remaining ones. Their importance should also be reflected. Then there's the potential of Helya's Val'kyr joining and Sylvanas still attempting to do something with Eyir's Val'kyr.