The only thing I care about with the new dungeon is whether or not it will be coherent with the plot.
If it's going to be a generic bonus 99F dungeon you need to have everyone at level ** to beat, then I won't be satisfied or interested.
It would be REALLY nice to turn Lavos' insides into a dungeon, but isn't the dungeon supposed to be optional? That would mean you would have to go through the dungeon in order to beat the game.
I think a good thing to consider is what time era needs a dungeon the most. So, here's the list of time eras and their respective dungeons:
Prehistoric:
-Mystic Mts.
-Hunting Ground
-Forest Maze
-Reptite Lair
-Dactyl Nest
-Tyrano Lair
Count = 6
Dark Ages:
-Terra Cave/Beast's Nest
-Mt. Woe
-Ocean Palace
-The Blackbird
-The Black Omen*
-Sun Keep*
Count = 4
Middle Ages:
-Truce Canyon
-Guardia Forest
-Cathedral
-Zenan Bridge
-Denadoro Mts.
-Cursed Woods
-Magic Cave
-Magus' Lair
-Giant's Claw*
-Sunken Desert*
-Northern Ruins*
Count = 8
Present:
-Guardia Forest
-Guardia Castle(prison)
-Heckran Cave
Count = 3
Apocalypse:
-Lavos
Count = 1
Future:
-Lab 16
-Arris Dome
-Sewer Access
-Lab 32
-Proto Dome
-Factory
-Death Peak*
-Sun Palace*
-Geno Dome*
Count = 6
* = Optional to complete the game
I grouped the dungeons that transcend eras into the time that is most coherent with their purpose. For example, the Sun Keep is grouped into the Dark Ages since that is where you first hear of charging a Sun Stone, as well as the fact that you are first introduced to the Sun Palace there.
Also, since I am biased in favor of the plot, Other things were grouped by relevancy as well, like Lavos being in 1999AD.
So, this leaves us with 3 eras in need of a new dungeon: The Dark Ages, Present and Apocalypse.
My vote goes for the present since you get stuck in Zeal for so long that adding to it would ruin it. I also think that making a 1999AD dungeon would require more elaboration in terms of the world map, NPCs, etc.
I'm all in favor of a Lavos shell dungeon, but that's probably not going to happen since it would be too perfect. D:
So, what amazing secret plot-coherent dungeon can we stick in Chrono's back yard? I have two and a half ideas:
1.) Something that would allude to the upcoming war between Guardia and Porre. Think about getting caught in a secret Porrean base, or even starting the war yourself! There's plenty of potential for some powerful enemies, maybe even a Dragon Tank MKII as a miniboss.
2.)Give Chrono his own red portal. I mean, he gets next to no character development in the game besides being a fearless hero that has no problem saving the world right from the get-go. Instead of performing a single task like in Lucca's red portal, you could have the party go through some kind of cloister of trials. It can be in order to prevent something horrible, like a baby Chrono from being killed by someone(maybe a decedent of Lynx? That would be an interesting link to Chrono Cross).
2.5)Either that, or make the portal something along the lines of Cyan's Dream (FFVI). It could be the first defective gate in the game and can allow the party to go to a crazy place, such as inside Chrono's head. I want to find out why Chono is the only guy in the game with massive spiky red hair and have a pure sky-elemental magic heritage. He had to come from somewhere juicy, and I think he is the most mysterious character in the entire Chrono series. Just anything where the party can float around Chrono's past (maybe a nightmare from when he was a kid?) and figure out why he was chosen by the Entity to be the real hero of time (Eat your heart out, Link).