Okay, I'm having an imagination-running-wild moment here. The opening scene/sequence as I currently see it (outlined)...
Main Character, another silent protagonist, is out doing something. (More interesting than collecting dragon scales.) Maybe checking out the ruins, because they live in Choras. Along the way, MC runs into Crono/Marle Jr. (who actually isn't the "main character") and they, whatever, relate to one another. Maybe they're best friends, maybe it's a love interest thing, maybe both.
They get back home as the sun is setting and see some unusual piece of technology (a mini robo, a car of some sort, doesn't matter), and wonder what's going on. Crono/Marle Jr. decides to sneak up and eavesdrop.
Inside is a purple haired stranger (Lucca, duh), telling of her dire straights in Guardia. The resistance is faltering, she says, but if they were to just come back, they could turn things around. The site of her and Crono... ("what," junior thinks, "my dad's name is Mike [or whatever] not Crono.")... would invigorate the entire kingdom.
Marle says it wouldn't be enough.
Lucca says she has a plan. She's been revamping her telepod. She no longer needs a second fixture to teleport to, etc. They could execute a daring plan to kill Dalton (or whatever), and put the Porre army in disarray. Blah blah, she needs their help, though. They know the castle, they can fight, etc.
Marle says that her magic has been fading over the years. ("What?" Junior thinks. "Mom can use magic???")
Lucca admits hers has too, but it doesn't matter.
Marle says she isn't the same person she was back then.
Lucca says you're a hero, and a fighter.
Marle says that's who I was. I'm a mother now. Look what they did to Daddy... (and play this song:
https://www.youtube.com/watch?v=qwi9cjQkteA). They killed him, just like they'd kill me, and Crono, and junior! Just to wipe out our bloodline. It's all so stupid, I want no part of it!
Lucca says her people are suffering.
Marle says I can't help that anymore, and they're not my people anymore. Junior's my responsibility now. I won't let them take him/her too!
Crono, meanwhile, is silent during this whole thing. Lucca appeals to him, and he drops his head in defeat and leaves the room.
Junior's like... what the shit? I thought we were simple common folk! Lucca leaves after giving them a good scolding, and Junior meets her outside, wanting to know the truth. Lucca tries to tell them it's nothing, but there is no dissuading. Still Lucca doesn't divulge anything. Junior pulls some kind of stowaway, possibly after confronting Marle about things, maybe just leaves a note, maybe both; and maybe Marle reveals who she and Crono were at this point (without the whole Lavos thing), or maybe she doesn't. Anyway, main character naturally follows...
Annnnnd that's how we start!