Suggestion:More Celtic Cultures

[Voxysmash]

Cruthin - Irish Picts Possibly Pre-Indo-European
Britonian - Formable by Bretons, Cornish or Welsh who Settle in Galicia
Gaulish - Gaulish culture only went extinct presumably 500-600 AD although there are attestations of people speaking gaulish till the 10th century.
Bonnaud maintains that while Latinisation occurred earlier in Provence and in major urban centrers, while Gaulish persisted longest, possibly as late as the tenth centurywith evidence for continued use according to Bonnaud continuing into the ninth century, in Langres and the surrounding regions, the regions between Clermont, Argenton and Bordeaux, and in Armorica. Fleuriot,Falc'hun, and Gvozdanovic likewise maintained a late survival in Armorica and language contact of some form with the ascendant Breton; however, it has been noted that there is little uncontroversial evidence supporting a relatively late survival specifically in Brittany whereas there is uncontroversial evidence that supports the relatively late survival of Gaulish in the Swiss Alps and in regions in Central Gaul. Drawing from these data, which include the mapping of substrate vocabulary as evidence, Kerkhof argues that we may "tentatively" posit a survival of Gaulish speaking communities in the "at least into the sixth century" in pockets of mountainous regions of the Central Massif, the Jura, and the Swiss Alps. if one takes over france as a gaulish culture they would get the "Indomitable Gauls" Achievement in Reference to Asterix and Obelix
Galatian - Formable by Gaulish

I think these would be a great and interesting addition to the game especially for the ruler designer

 

[Blk82]

Those cultures were dead by the 867 start date. Even by your quoted sources, the consensus seems to be Gaulish died out in the late 6th century, and more than 200 years before the earliest start date. If there were any notable rulers who spoke Gaulish, or Gaulish-majority provinces in 867, then few people would assert that the language died two and a half centuries earlier.

Adding four dead cultures for a minor culture group is silly, and such cultures would be difficult to play, as there would be no provinces and very few characters of that culture when the game starts.

 

[Voxysmash]

but it would be good for alternate history buffs to add some dead cultures like they added paganism a dead religion by this time....

 

[Blk82]

There are an innumerable number of extinct cultures. Including them all is not possible. Almost all of the extinct cultures in the game files persisted until at least the early middle ages, and most would not be out of place in CKII's earliest start date of 769. The "Roman" culture is there mainly for the history files, so Augustus isn't called an "Italian" or some other anachronistic term.

 

[Voxysmash]

I'm not asking for all of them just the more famous ones
Cruthin wasn't dead by the start date, it was the majority of the province of Breifne
Britonian is a formable culture from breton

 

[Aquamancer]

Thanks to the addition of Cumbrian, Cornish and Gaelic cultures, the only Celtic culture the game really needs in my opinion is Norse-Gael, to more accurately represent the assimilation of the Vikings into the Gaelic culture (representing the Norse-Gaels as Norwegians doesn't feel appropriate to me at all). As for your proposals:

• Several scholars seem to disagree on the assumption that Cruthin were Picts, based on archaeology, linguistics and recorded customs of the Cruithin all aligning with the Irish more than with the Picts. Not to mention that the Cruithin appear to have ceased to be an independent actor by 867. So, to me, Cruithin don't look like they differ from the other subdivisions of the Irish apart from the fact that their name has been linked with the Picts. Similarly, the notion that Picts are Pre-Indo-Europeans is a rather outdated notion, as most people nowadays seem to agree that Picts were Celts, albeit it is uncertain what their affiliation was with the Brittonic and Goidelic Celts.
• To my knowledge, the Britonian Celts in Galicia had assimilated to the Hispano-Romance culture of Northern Spain by 867 (as had the Cantabrian Celts of Asturias), so their representation doesn't seem particularly relevant to me (especially since there's no evidence of any Britonian Celtic aristocracy in 867, as far as I know). As far as I know, we don't even have records of their language, so we'd have to basically make up their namelist from nothing. Furthermore, having the Britonian Celtic culture as "reformable" seems dubious to me, since the culture would be "Neo-Britonian" rather than authentic Britonian.
• Gaulish seems to be the only culture you're proposing that would be plausible to implement, if Bonnaud's research is accurate. However, like Britonian, it would only be present as a provincial culture with no native rulers left, and historically at its deathbead. Besides, we're not even sure if the Gauls formed the provincial majority populations at their last few refugees in 867.
• Galatian language were last attested in 500s, meaning that it is most likely that by 867 they had been assimilated to the Greek culture or were such a small minority that they managed to avoid the attention of historians, meaning that they would've been such a small community that their representation in-game seems redundant. And as with your proposal for the recreation of Britonian culture by Bretons/Cornish/Welsh characters, the last surviving Gauls during the Middle Ages probably wouldn't have resembled their linguistical ancestors too much due to language shift, meaning that if the Gauls of 867 were to somehow take control of Galatia, the resulting culture wouldn't bear a close resemblance to the historically-attested Galatians of the Antiquity.

All-in-all, I don't think the implementation of long-dead or moribund Celtic cultures with no native rulers left would be particularly useful, especially since plenty of period-appropriate cultures are currently absent from the game, and should preferably be worked on first. Besides, if dead cultures should be added, the priority should be placed on the cultures of characters in history database who are currently represented with a wrong culture (such as various rulers from Antiquity and the Dark Ages). This way, players using ruler-creator have access to new cultures, while history files are made more accurate.