There has been much speculation regarding the fates of Crono, Lucca, and Marle at the time of Chrono Cross. Some believe them to be dead due to the ghost children and the fall of Guardia. Some believe them to be alive due to Lucca's letter, and others see those two things as contradiction and become indecisive on the matter. However, when all of the dots are connected the answer is really quite clear.
First off, let me say that they are both alive AND dead in CC. First lets look at the ghost children. They appear in the Dead Sea, Opassa Beach, and Terra Tower. In the Dead Sea, they are angry with Serge. They blame him for the future they prevented coming back in Home World. Many find this odd and wonder how Serge surviving could make that happen, but we will come back to that. Second we have Opassa Beach, where they appear to reveal some last minute plot twists and tie up loose ends before the final battle. Now the important thing to note here is that both of these take place in Home World, where the future is destined to be destroyed again. Terra Tower is in Another World, but you only encounter them in a small pocket dimension created by Belthesar and not Another World itself. This small pocket dimension exists in between both realities and the ghost children were pulled in temporarily, same as you. Upon exiting, the entrance is gone and you cannot return to it.
So what all of this tells me, is that they are dead in Home World, but when did they die? The answer is quite obvious actually. They died in 1999 AD during their final battle with Lavos. Home World splits when Serge is saved by Kid and many things are different from that point forward. CC takes place in 1020 AD, long before the Day of Lavos...so the Final Battle is now eligible for an alteration, which all of a sudden makes the Dead Sea and Serge being blamed a lot more believable. Crono and gang do not fight Lavos in 1999 AD until after they defeat Dalton on the Black Bird, which would still motivate him to militarize Porre and attack Guardia. He would have no way of knowing Crono already failed here. If Lucca had died against Lavos, she would have never taken in Kid either. This explains Kid's absence in Home World as well.
Now lets look at the letter from Lucca, which implies they are alive. This letter is given to Kid in Another World, which is the main timeline where the events of CT played out. This implies that Crono and Marle survived the fall of Guardia, and helped Lucca create an orphanage for war orphaned children. This is supported by the drawings seen in Lucca's house, as if they were around to help with the kids. What happens here is likely that they join a resistance to take down Dalton and begin rebuilding Guardia. There are a few factors that support this.
The fact that Porre wants the Frozen Flame implies that they need it for some reason. It does not confirm that Guardia is rebuilding, but it does show that resistance is being met somewhere.
In the general Kid ending, she refers to Guardia being her first target for world domination, then they would attack Porre. This implies that Guardia does exist at some capacity, but is weaker than Porre. Pick off the weaker nation first and attack the big dog second.
Radius says Guardia was once peaceful, which can be taken two ways. It was either destroyed completely, or they are at war. While it does not confirm that Guardia is still around, it does not confirm its absence either.
And now for the BIGGEST piece of evidence that not only was there a resistance, but Dalton was defeated. Guardia and Truce exist in 1999 AD, even after Dalton loses on the Blackbird. Thats right, the evidence is in CT itself. If you lose in the final battle (before or after Dalton), you see Lavos ravage the world. Not only is Truce (part of Guardia), Porre, and other nations mentioned in the destruction, but there is another dome that sits just above where Lavos emerges...right where Guardia Castle would be. This implies that somehow, after the fall of Guardia where the Castle was taken and Truce is burned to the ground, Guardia was able to rebuild itself as a nation. And in order for that to happen, Princess Nadia herself would have to at least survive the initial attack. If she survived, it is likely Crono did as well. This is again backed up by my other points regarding Lucca's letter and the absence of the ghosts in Another World.
What seems most likely is that Crono and Marle eventually defeat Dalton and rebuild Guardia. With Dalton removed, Porre could have been more diplomatic with his replacement and even helped Guardia rebuild in the name of peace. Crono's enemy was not Porre after all, but Dalton himself. This is how both Porre AND Guardia are present during the Day of Lavos.
Lucca being alive at the end of CC is a completely different, and much simpler matter to explain. The Chrono Cross merged realities and removed Chronopolis from the equation. This means that there is no FATE, which means there is no Lynx, which also means that Lucca is never kidnapped. This is supported also by the fact that Kid/Schala seems to live a much happier life at the end of CC.
And that is really the only thing that makes sense to me. There were a lot of dots to connect on this one within both games, and I am quite pleased with the results. So what do you think? Is this how you pictured their fates to turn out in the end? Does this theory sway you at all of not? And if not, how do you see it playing out?