This article is a companion of sorts to the Warcraft Retcons article. That collects retcons, while this article collects things that were simply forgotten about.
Just like that list, this is non-exhaustive, since I’m simply not aware of everything.
These will be sorted by whatever thing they were introduced in.

This collects elements that were forgotten about, seemingly abandoned or, perhaps, were accidentally left in from some previous development cycle.

The mur’gul are…murlocs? Uh…
The mur’gul are either murlocs, a variation of the murloc, a related species or something else, it’s not quite clear. They are 100% related to the murlocs but that’s about it.

They first appear in The Frozen Throne and work alongside the Naga (as slaves). Though it is implied that not all the mur’gul working alongside the Naga are slaves…but that’s if you make the assumption that I do which is that only the explicitly labelled ‘Mur’gul Slave’s are slaves and assume that all the other mur’gul (mostly the warriors) are not slaves.
Let’s be honest: it would be weird to have them as slaves, considering they are both smirking and on the front lines.

It’s notable that their description from the RPG is a bit different and, potentially, far more interesting.^A)

The mur’gul have not re-appeared in Warcraft since The Frozen Throne which is quite odd, though not if we consider them being simple murloc reskins. Why they were ever made in the first place is a mystery to me, though. Like…the Sea Witch used murlocs so, like, what?

Mur'gul

The Ogre Legion is a mysterious faction that existed(?) in Reign of Chaos. The only evidence of their existence is flavor text for the Ogre units in the game.
The page I link below includes some speculation as to what they were/are and what the intent was.
I’m leaning towards it being a remnant of a plotline from a previous version of the Warcraft 3 storyline, though that doesn’t change the fact it’s still in the game and has since been forgotten about.

Ogre Legion

In Warcraft III, the highest-level succubus unit you could face was the Queen of Suffering. She re-appears in the RPG, but has since disappeared from canon.
I’m sensing a bit of a trend here with the RPG.

Queen of Suffering