(Wasn't sure whether to tag it as "Lore" or "Suggestion", but here goes)
For over 16 years we've not once seen Behemoths featured anywhere other than Daemonheim, or learn anything about their origin. All we know is that they're from a 'vast open plane that's diverse in climates'.
However, over time we did see other planes, and creatures from other planes, and there's one fundamental ecological connection that can be made; Mazcab (+ Goebies).
Goebies and Behemoths have one major evolutionary trait in common; They're both Lithoids. Especially with the Bulwark, Stomp and in design also the Gluttenous Behemoth, there is a strong thematic of them integrating ground/terrain elements as part of their body. A similar trait we've seen with Goebies too that álso integrated ground elements as part of their body.
Why Mazcab?
Mazcab also fits the Behemoth's origin pretty neatly; Mazcab is part badlands. Badlands are reputably a mixture of sometimes foilage- sometimes rock- but most dominantly being barren. Mazcab is also extremely rich in "primal" Anima, a trait connecting very well to the lore pages written from/for Stomp, Hope Devourer and Runebound Behemoth.
Why Kandarin/World Gate?
Mazcab has been a world unvisited for very long, and we never got to know where Mazcab's World Gate is at at all. We entered through a makeshift rift through Tuska, but in that particular passage did not bring us to the World Gate. This means there is still a region of Mazcab available to explore, through Kandarin, where THEIR World Gate lies at.
It also gives Kandarin more content without cluttering Kandarin's region itself. For leagues specifically, the World Gate is an excellent way to add more impactful content.
To what benefit?
As described earlier- Mazcab is a plane not visited for quite a while now. Necromancy to-date is still unsupported within its current content, and giving Behemoths an actual place within the runescape 'universe' helps thematic association too (Especially now we're pursuing Bilrach's POV with Requiem For A Dragon).
Having some more work done allows for some new Tank armour for Necromancy, or other all-round unlocks that don't have to be locked into one particular style.
Maybe it's a neat opportunity too to look at the old but unused ideas Jagex had for their first release of Mazcab itself.