I had an idea about how to make that work.
Imagine a new category of power sets that combine defense and support powers. Just like how Dominator secondaries combine melee and ranged dps powers. The new AT takes its primary from this new hybrid category, and its secondary is melee dps.
Basically take a defense power, and replace most of the toggles with AoE buffs. Mix and match powers from a matching Defender primary as needed. So the set is a mix of self-protection and team support. This is easiest to imagine with defender powers that give out heals, defense, or resistance buffs, rather than debuffs. But there are a lot of defense powers that give you a debuffing aura.
Energy Defense: Energy Aura & Force Field -
Ice Defense: Ice Armor & Cold Domination
Healing Defense: Regeneration (or Willpower) & Empathy
Fiery Defense: Fiery Aura & Thermal Radiation
Some defense power sets don't jell as well with their support analogs, and would take more work to integrate.
Dark Defense: Dark Armor & Dark Miasma
Electric Defense: Electric Armor & Electrical Affinity
Radiation Defense: Radiation Armor & Radiation Emission
The AT would possibly be a little tricky to play, since you have to keep restoring the buffs on yourself to stay tough enough to survive melee, while also keeping an eye on your allies.