If someone (re-)played WC3 TFC and compared its world building with that of WoW's, they might notice some curious detail: in the Frozen Throne a new "subspecies" of murlocs was introduced, "mur'guls", who were never elaborated into pretty much anything later.
Admiteddly, as the wiki tells us, mur'guls are considered "normal murlocs". And yet, they are sufficiently distinct compared to ROC murlocs: "mur'gul differ from murlocs in that they have larger teeth, armored scales, and spikes on their backs". While some of that design carried over (like adding "spikes" to their spines), it still raises the question - what exactly were mur'guls meant to be?
Are they simply a regional variant of murlocs? A tribe altered by the nagas into servitude? The Warcraft RPG offers one interpretation, describing them as savage creatures who "fear only one race: the naga of Nazjatar", while their origins are portrayed as speculative: "some say that these creatures were once murlocs, but the Burning Legion's powers cursed and twisted them".
Considering TurtleWoW's approach to older fragments of lore, it does feel like an opportunity to expand upon. I think there are several potential directions this expansion could take: if mur'guls are just a regional variation of murlocs, then perhaps the murlocs in Kalimdor (like those in Dustwallow) could be reinterpreted as mur'guls, replacing their old names and changing quest text to reflect the shift. They would also need a new model, which might emphasize a "leaner" build, replace fins with bony spikes, while their texture could make them look more "scaly" - closer to nagas than murlocs.
Alternatively, if they’re more than a simple regional offshoot, they can be connected to existing (or potential new) naga or Old God-related content. For example, Blackfathom Deeps' murlocs could be remade into mur'guls (curiously enough, retail WoW has added "Deep sea murlocs" there. It's not explained what exactly the origin of those murlocs are, so they might be mur'guls too? I know that for TurtleWoW purposes it is not canon, but at least reusing that model might save some time making new one).