I have recently been playing around with the Celtic roster, a wonderfully powerful bunch of warriors - almost too powerful except for one thing, most of their key units are not available in TC.
Saethwr, their only missile unit - Roundhouse and Archery Range only.
Woad Spearman, their only anti-cav unit - Roundhouse and Barracks only
Celtic Chariot, their ace in the hole - Roundhouse and Stables only.
So to field a full Celt army, you need more of your factory slots taken up by Roundhouse, Archery Range, Barracks and Stables, therefore fewer Advancement Centres, Siege Workshops, Docks etc. End result is that the power of the Celts is offset by necessarily being less technically advanced, a very interesting trade off.
Axe Throwers are an exceptionally powerful unit too, a combination of their cheap cost, versatility, high bonus from range Vs Siege and Structures, and their buffability due to being ranged mêlée. Axe Thrower spam is a crazy powerful defensive tactic (trust me, I know. I used to do it until I got bored of how easy it was).
How about off-setting their power by having them trainable in Barracks and Norse Hall only? That way it requires more investment to benefit from their immense power.
Let the backlash begin!