You are magnifying everything.
For every 3 civilizations you say that are missing in America I can bring up 6 that are missing in Europe and Asia.
So what if some are missing? You can't include every civilization. Just the very dominant ones.
And these are the Aztecs, the Maya and the Inca.
You can rightfully say that the Toltec or the Huari were indeed important and influential, but we don't have enough information about them and there are no known leaders of them, so they are in a lower priority.
I think that civilizations like Afghans, Kushans, Tamils, Burmese, Khmers, Gokturks, Hittites, Seljuqs, Xiongnu, Tibetans, Vietnamese, Bulgarians, Venetians, Congolese, or Malagasy are much more important AND documented, and also can represented a not fully represented area (except the Khmers and Venetians), those are in a higher priority to me than some more Mesoamericans.
Adding more of those Mesoamericans that you mentioned are similar to adding the Akkadians or Elam to the current game - they were important, but people will rightfully say - "there are greater empires for their spot, their area and their culture is already fully represented."
However I do support adding more CS like Teotihuacan (which is a classic CS).
Please, don't compare Mesopotamia to Mesoamerica.