I do hope the Celts come back as a single entity. Sure there were many different tribes but does a tribe qualify as a "civilization"? They had enough socio-cultural similarities to be remembered by history as a single group.
That's just the Hellenistic bias of our history. The Greeks also saw everyone inhabiting the Levant as a single group--NB they called Aramaeans "Phoenicians." The Celts were a group of cultures, not a single culture; they didn't worship the same gods or even speak the same language. Gaulish would not help you talk to a Celtiberian or Gallaecian or Briton. Even "Cisalpine Gaulish" is increasingly being recognized as not directly related to Gaulish.
there are still people today that identify as Celts
That's just nationalism. Yes, Irish, Gaelic, and Welsh are Celtic languages. They're also Indo-European languages. Maybe we should have an Indo-European civ that covers all of Europe, Greater Persia, and northern India conveniently.
A single Celt civilization would have more options for uniques and could open more civilizations in the later eras. Also with Civ7 multi-layered special abilities, it would certainly be possible to design some uniques that reflect the diversity within the group.
I'd consider this a compromise of the Mississippian civ, not a bonus. We simply don't know enough about the Mississippians to make a more specific civilization. We do about the Celtic peoples. We've already had the sort of Celtic civ you describe in Civ5, and frankly the Civ5 Celtic civ is one of the most patently offensive mishmashes to come out of Civilization. There's nothing a Celtic civ could bring to the table that couldn't be adequately represented by a Gaulish civ. Quickly sketched example:
Isarnon Gallika: Gauls gain +X Gold for every Empire Resource they have access to. Gauls gain +X gold from pillaging.
Unique Building: Nemeton: Happiness base; gains Culture from adjacent wonders, lakes, navigable rivers, and mountains.
Unique Building: Briga: Production base; gains Gold from adjacent resources. Must be built on rough terrain.
Unique Quarter: Dunon: Counts as a Fortified tile; gains +1 Culture and +1 Influence for every trade route to or from this city. Allows building Druuides.
Unique Military Unit: Ambaxtoi: Unique heavy infantry wearing chainmail and carrying a heavy shield and spear. Gains +2 combat strength when defending; can create fortifications in half time.
Unique Civilian Unit: Druwides: Each Druwits may only be built once, etc., etc.
Best of all, no references to nonsense like "woad warriors."