I went with an archery theme, actually inspired by one of my favorite M:tG cards. The unit is called the Sagittar, and uses the Ljo Longbowman graphics instead of the regular FC graphics (I have zero skill at doing artwork, so...). Instead of the Siegecombat promo, it has a similar promo that does basically the same thing but additionally allows it to promote to the Archery promos. I also decided to have it start with Woodsman I; was considering allowing it to promote to Woodsman II, but that seemed too much.
Ranged combat wise, it loses the ability to deal collateral damage, but has the potential to get a much higher ranged attack power against single units, thanks to the Archery promotions. I also gave it a 2-square range, which adds another layer of strategy to deciding where to place them, since you want to put them somewhere where their full range won't be blocked by high terrain.
"What's their strike range, you ask? Let's put it this way: sagittars aim their bows using maps."
Otak, Tin Street shopkeep
Originally I was going to give them a unique replacement for the Commander I, II, III line of promotions that increased their range as they leveled up - starting them with normal 1-square range, bumping it to 2 at Commander I, and possibly even up to 3 at Commander III (in my version I enabled Commander III for everyone, instead of just the Kuriotates). However, I discovered that there is currently no way to increase ranged combat attack range through promotions, which struck me as a bit odd.
I've also done fort commanders for the Sheaim, Bannor, and Chislev so far if you are interested in hearing those; perhaps in return you could share any ideas you have for a FC for the Lizardmen, Dural, or Archos, because those seem like they should have them, but I am completely stumped as to what they should be. XD