The Celts are, maybe, the most complete factions of the entire game, and are all somewhat similar, like the 3-4 roman factions in Rome vanilla. As such, I could do only minor changements.
The Celtic upgrade is mostly based on the naturalization of "foreign" celts. While the Celts climb the ladder of their reforms, they become capable of persuading other Celts to fight for them, not as mercenaries, but as warriors of that same state (thus, many types of units from the Regional Barracks pass to the faction's Army Barracks, as the reforms proceeds).
AEDUI
Aedui and Arverni are the mainland celts, they share Gaul and, ideally, the entire area of the Celtic expansion.
The Aedui would probably be the one more directed to the British Isles and the Mediterranean Europe. As such, they would probably be the more powerful celtic power in the western europe, if they not clash too early against the Roman power.
With the Time of Bondsmen, the Aedui begin to assimilate in their factional army the British peoples, such as the Siluri (gaining the Siluri Warband) or the Goidilic (Goidilic Light Infantry and Goidilic Spearmen Levy), the Caledones (Caledonian Noble Cavarly). On the mainland, they can assimilate the halfbreed Massilian Hoplites, or in Celtiberia, the powerful Celtiberian Heavy Infantry.
The process further improves in the Time of Soldiers: most of the abovementioned units are still recruitable, at even lower level of the Barraks. In Hybernia, the Aedui gain the Goidilic Cavarly, while on the mainland the can recruit even the Arverni Nobles, and making Aemilia a celtic land again, they can reverse the Boii migration, making them available again in this land. Furthermore, they assimilated even the most eastern celts (like the Lugians) and the half-breed (the Gallo-Thracians). Lastly, the Galatians and the Egyptian Celts (the celtic tribes the Ptolemaioi "imported" and used as mercenaries), these tribes can be used again, if the Aedui make their way in Galatia, Odrysai and the northern Egypt, the elite warriors of these tribes are re-enlisted as Gallic Heavy Swordmen and the Galatians nobles as Celtic Noble Cavalry.
But there's more, the Druids. While still available with the Level 5 regional Barracks in all Britain, they can be recruited in the Isle of Darkness (Cambriae) if you build ANY type of temple at level 4 or more.
Lastly, a couple of southern populations that could give an hand to the Aedui: the Iberians (Iberian Light and Medium Cavarly, Iberian Medium Spearmen) against the haughty Carthaginians, and the Samnites (Samnite Spearmen) against the powerful Romans.
ARVERNI
Aedui and Arverni are the mainland celts, they share Gaul and, ideally, the entire area of the Celtic expansion.
For the position of their northern territories, the Arverni will probably direct themselves in the german area and from there, in the Balkans. Aside from the Aedui (and later, the Romans) the main enemy of the Arverni will be the Sweboz.
With the Time of Bondsmen, the Arverni assimilate in their factional army the eastern celts (Lugian, Galatians, Gallo-Thracians) and even some Goidilic units, if they succeed to conquer the Hybernia. The elite Galatian warriors (Galatian Heavy Spearmen) are re-enlisted as Gallic Heavy Swordmen and the Galatians nobles (Galatian Heavy Cavarly) as Celtic Noble Cavalry.
In the Time of Soldiers the process is further improved: most of the abovementioned units are still recruitable, but at even lower level of factional Barraks, with useful additions: in Britannia, Siluri and Caledonian Nobles; in the south-western area Celtiberian Heavy Infantry and Massilian Medium Hoplites; in the proper Gaul the mighty Carnute Warriors; in the northern Egypt (assuming they ever decide to set foot there) the local Celts can be re-enlisted as Gallic Heavy Swordmen.
Like the Aedui, the Arverni can still recruit Druids with Level 5 regional Barracks in all Britain, but now they can be recruited in the Isle of Darkness (Cambriae) building there ANY type of temple at level 4 or more.
Lastly, there're the regional additions. Being so close to the germans means the Arverni can talk with them more often than others, even if not in friendly terms. That said, for them is easier to enlist germanic units -after subjugating them- or even better enlist germans in half-breed units with the local celts. So, in the germanic area the Arverni can recruit Chatti Spearmen, Chauci Spearmen and Cherusci Swordmen, while the recruitment areas of Celto-Germanic Spearmen and Celto-Germanic Cavalry, is broader.
In Thracia, near the local celtic settlement, the Arverni can secure alliances with some local tribes, thus gaining precious the Triballi Infantry.
CASSE
The Casse concentrate primarily on the area of the North Sea: Great Britain, Gaul, and other coastal areas, primarily Gallo-Germanic.
If they could unite the British Isles, they would find themselves in a peculiar situation of proto-Vikings, able to carry out raids on all the surrounding lands, almost with impunity. And it's there that they stand now.
With the Time of Bondsmen, the Casse begin to assimilate in their factional army the lesser British peoples, such as the Siluri (gaining the Siluri Warband) or the Goidilic (Goidilic Light Infantry and Goidilic Cavalry). On the mainland, after having subdued the Aedui and Arverni's kingdoms, their champions (Carnute Warriors and Arverni Nobles) can be persuaded to fight for you, in the armies of the Celts came back from overseas.
This process improves further in the Time of Soldiers: the mainland Celtic champions are still recruitable, but is increased the absorption rate of the British peoples, they become available at even lower level of the Barraks. On the other hand, establishing such a great Celtic state means the total assimilation of even more reluctant elements, as the Goidilic nobility (Goidilic Noble Infantry) or rare and almost mythological tribes (Ebherni Armoured Infantry).
Regarding to the religious sphere, certain temples, in particular areas, can attract groups of religious supporters that, trying to honor their gods, end up providing useful troops. This is the case of the Cromm temple, in Hibernia, which at important levels musters the rare but powerful Goidilic Shock Infantry, men armed with heavy two-hands hammers. Or, by the time chariots have become (theoretically) obsolete, supporters of Llew (Celtic god that exemplifies the Celtic virtues) will assemble near great temple of such divinity. Here, the they will train at combat like their ancestors did, thus providing units of Celtic Chariots, in some cases strategically important assets.
In the end, looking beside the Celtic lands, a possible prey would be the near Scandinavia, where it will be possible for the Casse to recruit some useful Germanic units (Gaut Spearmen and Germanic Club Infantry), reliable although not overly powerful.