Hello,
I'm glad that you picked up Mighty Armies. It is a great game imho, and i've been playing it for a while. I've hacked a lot of my own custom rules onto the game and added units. I more or less try to base custom units on existing ones, as there isn't a table for point costs.
One unit i have made for barbarians is Frost Giant. I use an old Ral Partha figure, it is HUGE compared to 15mm scale. It's stats are move 5", Fighting +7, Support 0, Fearless, Rampager, Shooting II, cost 7 AP. I made its support 0 to balance it. Giants are arrogant they wouldn't help puny runts. It can't be grouped with other units. It is is more or less immune to most shooting like arrows, but cannons and trebuchets can kill a giant, as well as hand guns (I'll maybe get to that another time)
What do you think? I have used this in my own games, it is quite a lot of fun. The shooting I treat like a dwarf cannon, except the giant hurls stones instead.