Yes, I'm doing things different here than what you'd normally expect from a Chrono Trigger Let's Play. First, an explanation for those new to the game.
Chrono Trigger has multiple endings. A lot of them, actually. Exactly how many is complicated, but generally you'll get the "default" ending first, and then unlock New Game + to get the rest, though technically this is the only ending that I'm pretty sure requires New Game + to access, since that mode adds a warp straight to the final boss to the right Telepod.
I plan to show off all the endings as they would happen in-game were you to skip to beating the final boss (actually I just lied there but I'll get to that). Obviously I'm not going to show any of the final boss yet, but generally these endings are surprisingly not that spoiler-filled for the point of time they show up, and I think it'll be more novel if I showed them exactly where they'd happen within the story.
There is one exception, the one that you can do at the very beginning of NG+, but that will very likely be the final update of this LP due to what exactly that entails.
For now, enjoy our first ending of Chrono Trigger!
Full Video: The Successor of Guardia (Ending) 
Guardia Millenial Fair 
Only Crono and Marle are here. Lucca leaves the party as soon as you get back to 1000AD, so you can only fight the final boss with the other two.
A lot of characters have their dialogue changed, and also utter this weird "ribbit" sound effect, pretty much identical to Frog's.
In fact, most characters will say this same dialogue, though a few of the more-important fair patrons will keep their dialogue.
There are two other variations, also with the ribbit effects.
Otherwise the fair is semi-normal besides the two men at the entrance.
Let's return to the castle!Next, we'll meet Marle's father, who pretty much looks exactly like King Guardia from 600AD.
Courage and Pride 
Where have you been, Nadia? First, calm down. And why those clothes? Everybody keeps saying that! What do you mean? What ARE you babbling about? You needn't keep dressing that way!
So...you may see where this is going...
SILENCE 
That's right, they had movie technology in 600AD. A wizard must have done it.
Here we have Frog and Leene...
...inside the Cathedral...
...and they're...married...
Apparently King Guardia XXI dies as a result of our actions in saving Leene, so she marries the only other person who cares so deeply about her.
So yeah, our first ending results in a queen screwing a frog
Yikes.
