I wanted to promote the idea of a Celts split, without intending this to be a priority over anything else or exclusive to them.
The Celts Civ is just one of those umbrella Civs that have much more entropy than they express. A split could bring that out, possibly with original ideas gameplay wise. There still is uncharted territory in that sense.
I’m not going to be specific about this, I don’t even need to, but there are a few ideas to work on. The split would be, as by title, Irish, Scots and Welsh.
Scots
Probably the Civ that could inherit the most from the Celts and be the free one for that reason. Below average cavalry and archery but inherits the siege bonuses and keeps a focus on infantry.
Surely has to feature Highlanders, maybe as a Two-Handed Swordsman replacement, with a charged attack. It’s actually a system pretty much like that of the Romans but with different stats, I suppose.
The UU could be a pikeman, since the Scots developed sound tactics with pikes, the issue though is the risk of overlapping with the Kamayuk.
An already made Civ concept:
Irish
The Irish could be the ones to inherit the wood gathering bonus and the one for the faster infantry. A difference could be having good Monks and a bonus for them. A sea bonus also? Faster moving Fishing Ships that double as small Transport Ships?
The Civ could represent the raiding and fighting amongst clans but should depict the figure of the High King and its authority.
For them I imagine making the Skirmisher a much more general unit, possibly substituting the Archer line altogether. Could be replaced by a unit, the Kern.
The UU could probably be the Woad Raider or something similar, localised. Still fast melee infantry.
Welsh
This is tricky since the Britons already take the famous Longbowman. But it’s not over yet.
The Welsh surely get the sheep bonus from the Celts. Other than sheeps, the Welsh are clearly an archer civ, that knows how to use the terrain for quick attacks and holding their ground.
To reflect their archery, they might have a special treatment for the Archer. Instead of having the Crossbowman and Arbalester, they get staggered improvements to the Archer as a Civ bonus, maybe reinforced by an Unique Tech.
As a Unique Unit I was thinking of a heavy spear unit to defend the Archers and provide a good first line, with some use against buildings too.