I haven't played in a while; are their single-edged Germanic swords? Might be nice for a late or high-tier unit, if those were in use yet. I'm pretty sure they were around by the 1st century BC. Celtic short swords would be nice too, especially up in Britain, with scarcer iron and so forth they'd be more cost effective, and in Ireland there were relatively few longswords (though they were present, just, they'd almost certainly been the province of nobles), but plenty of short ones (more or less large daggers, with broad flat blades, somewhat akin to a gladius in appearance). Maybe old-style Celtic longswords for the isles too, shorter with nice leaf shapes. For that matter, Iberian gladii, attenae swords, and other swords would be nice.
I'll reiterate some desires and list off some others. Fire-hardened javelins/spears with no metal (good for some of the more primitive or fringe places). Celtic clothing that isn't all the same color (a waistcoat and plain color shirt with plaid trousers would be nice), and removal of some of the bland vanilla-looking things from them (Celts were extremely colorful people, it'd be nice if they looked the part), correct one of the axes (it appears to be swung backward), more spear varieties for barbarians, and a general increase in barbarian equipment and units (it'd be nice to have roaming bands of wild Caturiges attacking merchants in the Alps, or Ligurian war parties fighting both the Gauls and the Romans).