Well if we go with the lore of Praetoria there are several ways a new AT could come about.
Tellurian Plague; a mix of a dominator/defender with DE like attacks, debuffs, holds, especially with "plant" based attacks.
Seed pods, spore burst, a spirit tree like power that is a aoe debuff, ranged plant attacks and melee attacks with a DoT "infection" debuff. With an inherent called Infection that reduces resist that stacks the longer a fight goes on.
Midnighter; a mix of blaster/controller with magic themed powers based through an item ie; a wand or staff with different choices in CC. Powers could be designed after in game attacks ie; fire, lightning, force, or psi.
"Darkened Path" would be a travel power just for this AT, a TP like power like the paths in 1st Ward it could do cross zone travel and to mission door.
Inherent would be called Magick it would dependent upon origin so if you are science, technology, or mutant enemies you get a bonus vs those origins that stacks.
Natural, and magic would get a bonus that doesn't stack.
Soldier/Seer a branching AT just like Veats, that branches into two vastly different choices.
Seer would be a blaster/MM type with ranged psi attacks and MM pets. Inherent would be Precognative a higher base defense and recovery.
Soldier would be a melee/blaster with attacks centered around their rifle, with blasts but also bayonet and rifle butt attacks, with armor that is S/L and psi resist. Inherent would be Strengthened, has higher base resists and regen.
Was also thing thinking about an AT called the Created, which centered around the Ghouls.
Would be melee all the way with a higher base regen and resist armor. Inherent would be Survive a stacking regen based on team size and if a team member dies it would be a Vengeance like bonus to the Created, just like when ghouls die they give heal/buff to other ghouls.
Just my ideas, our VG has been doing missions in Praetoria and we are in 1st Ward now. My controller is based on a Tellurian plague infected person, and I have obviously been thinking about Praetoria. In all seriousness, like @Piecemealis the dev for Villains, I wish I was the dev for Praetoria lol.
I'd love to created a zone for Hamidon, missions, arcs, etc all related to it. But, I have zero programming skills, just ideas, .....sigh lol.