Any idea could be stretched into a full expansion if you really wanted. One could for instance easily imagine a full expansion set only in a larger version of each capital for instance, filled with dungeons, raids and unique things to discover.
What we see in SL though is that just because you can doesn't mean you should. And that is exactly the thing with Argus.
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Looking back I think Argus could have been half an expansion. Mac'aree for 7.2 and Antorus for 7.3.