People need to learn the difference between a "civilization" and a nation. The reason that not every single individual nation that ever existed is found in the game is obvious. Wales is part of Celtic civilization.
Arguably, some of the picks in the game represent nations and not true civilizations (eg England or France) but I think if you look at the leaders chosen, its most often to be representative of a civilization in time which would otherwise have a weird name. France sounds better than the "Western Medieval Europeans" just as England sounds better than "Imperialist Europeans". Though I totally agree that "Britain" would have been a much, much better choice for a name. Along those lines it would be fitting to have two Celtic civilizations, or perhaps even three - one to represent the ancient Britons as well as the Gaelic nations of the UK, one to represent the Gauls, and one to represent the Celtic golden age (Hallstatt and La Tene cultures).
As to America, Lincoln in a fur cap bothers the hell out of me too, but on the other hand, its not any weirder than a Stone Age Queen Elizabeth, or Hammurabi sitting in an office tower. The absence of America would be conspicuous, I think, because it has profoundly affected the modern era and it is certainly distinct culturally. I used to think the same way about the States being in the game but on really reflecting on some of the other oddities that come up with civilizations out of their time, it isn't that strange. Medieval Iroqouis ... Aztecs with nukes ... a Roman Army made up of warriors in furs with clubs ... and Lincoln with a fur cap. It's all part of the game. To even contemplate having everything historically correct means no Epic Game, but only scenarios.
I must admit I do think they made a mistake when they got rid of breakaway civs that would happen when you captured a capitol in previous versions, though.
As for this:
I think the Americans should be in there but possibly as a super hybrid tribe which develops over the centuries incorporating units from all sorts of other civs.
No. All civilizations do that, to greater or lesser degrees. The Celts got their chariots from the Near East, the Romans got their triremes from the Carthaginians, and just about everything that was ever 'British' was brought from somewhere else - even the Celts weren't the first people to live there. We write with an Arabic script, and everyone judges time and radial degrees in sexadecimal numbers - a Sumerian invention.