That's the appeal of the army. Similar to FEC they have a small, yet flexible, model range and flavorful rules. All they really need are some updated rules for the Magmadroth and, ideally, make each hero on Magmadroth be more distinct. Other than that I enjoy their smaller unit roster since each unit has a place + each lodge and choice of general can take the army in different directions. With the new hero coming with the FS vs IDK box I think the standard battleline unit Vulkite Berzerkers are going to have more play. I won't say no to new options though, like a flame cannon or magma golems of ogor/monster size.
That said, I could see how CoS Duardin could slot right in with FS without compromising the army's identity. KO and FS would lead too much compromise in army design, i.e. limit builds into two tracks effectively killing what make each army unique. I would, for instance, hate to have one sub-allegiance be the Fyreslayer one and have next to no identity beyond that. Going back to FS + CoS Duardin though I can see it work both mechanically and lore-wise. Let's say Grimnir is restored, leading to duardin across the realms returning to renew bonds, swear new oaths, and rebuild the duardin kingdoms. GW have also showed they can do a really solid tome with a healthy amount of choice with SBGL.