I generally agree with you, but...
...with a few exceptions (notably the Mississippians) Civ7 has shown a preference for being specific with the name, sometimes overly specific, even where such specificity is jarring with the civ design (Prussia and Mughals both spring to mind). So I imagine we'll see Gaul again. (Here it's not even counterintuitive since the Gauls were essentially the apex of Antiquity Celtic cultures, and I don't think any of the others would really bring anything to the table except city names. Which, if Prussia is any indication, is something they can bring anyway...)
I agree with the specificity, and I'm not saying Gaul
won't be what we get. I certainly wouldn't be surprised to see it return as the general representative.
However, between the likelihood of the game wanting Celts/Picts/Scots representation for the British lines, and possibly even at least some suggestions if not outright representations of La Tene/Halstatt cultures if we ultimately see a Switzerland modern civ...I think there are several ways they could design the Celts in antiquity.
And yes, I think the obvious/easy design would be a Picts/Scots civ and a Gaul civ, but (1) I'm not sure if it's worth mechanically parsing those two out from each other or blobbing them together, and (2) if it's fair to give those two regions express representation while the Halstatt culture gets doubly shafted. I would also argue that if we get the "Goths" and the "Norse," (instead of Visigoths/Ostrogoths or Norway), there would be a certain elegance to just juxtaposing them against a single "Celts." Although I do recognize that, like the Khmer, the devs have limits as to how far they want to stretch back into reconstructive history and perhaps Gaul and/or Scots/Picts is where the happy balance will lie.
I'm not super concerned about how it will shake out, but I don't think we can discern a clearly "best" design at the moment.
Not sure what you mean by that unless you mean at the start of the game all civs are "tribal" like with warriors and capital cities being no more than a larger version of the barbarian encampments mainly pumping out military unit?
Timeline wise it doesn't fit Antiquity, but gameplay wise they could always make Great Moravia the progenitor Slavic civ.
They could but Gaul could easily just progress into a number of Western European civs too.
Possibly something like that, yeah. I am not sure how compelling a prehistoric age could be made mechanically, it might not have much potential and be fairly boring. But I do think for some civs such as Scythia or Sumeria a nomadic/pastoral period with fewer "requirements" such as city-lists, associated wonders, etc. could be a lot more forgiving of design than trying to force them into a standard civ model. I do think it would shake out as as kind of "IP-plus" gameplay. I don't know, I'm not wholly convinced it could work, but I'm also not convinced it
couldn't work yet, either.
My understanding is that Slavs could be pushed back just as far historically as Moravia. Although I wouldn't mind Moravia by any means, maybe even we could see a sort of blobby Slavs-Moravia civ in the same way VI had Celts-Gaul-Belgicae. Just like how that was Belgium's representation in VI, maybe a very Moravian Slav civ or a very Slavic Moravian civ could be Czechia/Bohemia's representation in VII.
(for this reason, I would not be surprised if we see "Belgium-as-Celts" return in VII again, depending on how the devs decide to assign civs across eras. The obvious counterpoint is quite similar to Moravia's situation: will we get Belgicae/Moravia in antiquity, or Burgundians/Bohemia in exploration? I doubt we will see both.)