Golden Girl

Or maybe something like the new zone-event-spawning-a-GM-spawning-an-instance style that Lord Winter's realm introduced to the game?

I'd already suggested something like thtát as a revamped normal zone event - like for the Steel Canyon fires, the Hellions are using the fires as part of a ritual to summon a fire demon - you'd defeat a set number of arsonists, which would unlock an instance of the burning building - inside you'd havea time limit to destroy altars/pentagrams/glyphs that they'd set up - once you'd done that, a GM fire demon would spawn, which you'd also have to take down within a time limit.

Or the Troll rave in Skyway City - defeat the ravers outside to spawn an instance of the rave warehouse - inside, defeat Supatrolls and destroy drug stashes to spawn a huge, mutated Supatroll GM.

Or a new one in Brickstown - an Arachnos Flyer would spawn and drop Arachnos soldiers into the Zig - once you'd defeated enough of them, an instance of the Zig interior would spawn - inside, you'd have to defeat various generic Villains like the ones we meet in the Safeguard missions, and then back outside, we'd have to take down the Flyer.

Here's something from the PAX Q&A that I've not seen mentioned yet, when talking about how GR zone designs are made to tell the story of Paretoria:

Reichsman was originally captured and kept in suspended animation in the Freedom Phalanx's main base in Boomtown, which was then destroyed/buried in the first Rikti invasion.

Reichsman's new base is described as being under Boomtown during the BTF, and it might even be built from the ruins of his former prison.

The 5th Column vs Council fights feature the 5th Column mocking the Council and their base on Striga Isle, saying their base is in the heart of Paragon City itself.

I think the chances of a Boomtown revamp as part of the evolving 5th Column storyline are pretty high.