Well,the problem with Khmer is the proximity with Siam,which is the similar reason why Mali won't come into G&K expansion . For Sumer,I think they had better chances to return when they released "Wonders of Ancient world" DLC,which didn't happen .
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I guess the devs' planning about this expansion was to reveal the "veteran civilizations(that had been in civ series before)" first,then reveal civilizations that have their first time in Civ series later . The main reason to sustain this is the fact that the first 5 civs were already veterans and one of the last 4 remaining civs which had been spoiled unintentionally(huns) is a new Civilization .