The Celtic Leaf spear, in brytenwalda it was two handed and sorta OP. But anyway it looks like this (Couldn't find an in game picture)
http://www.outfit4events.com/runtime/cache/images/productFull/pon_0503.jpg
Whats special about it is that it does penetrate damage with the thrust and slash damage with the swings. Its sort of like a spear version of a two handed axe, less damage but quicker and good for an agility build. Heres an awesome video of it being used on a gel head in real life.
https://www.youtube.com/watch?v=tcM3l2aBTCg (Awesome overhead swing at 6:06)
It certainly could have been around in this time period since it was in the iron age before the viking age, that these were more widely used. Perhaps there could be a special tier of high-end Celtic troops that use these (or add it into the rotation of weapons for spearmen). You could also have some rare version of the weapon that has better stats and a nicer texture, which could be a cool Celtic relic with a little side quest or something.
If you don't think its historically accurate then you could only put in the rare version and no generic ones for troops to use, like you did with the troll axe which is a bit crazy.
A lot of people only use spears on horse back and this gives people a spear which is more practical and serves the role of a two handed weapon for an agility build. I think there should be some more weapons and this is IMO the perfect addition.