Actually, I developped a theory of my own (it was thanks to this theory I found out about this site):
(By the way, sorry if you don't understand what I say sometimes, it's because english is not my native language)
Serge is Lavos.
Yeah, I know it doesn't make much sense right now, but keep reading...
Well, somewhere (I can't recall exactly where right now) you are said that whoever touches the Frozen Flame either burst into flames himself or becomes something different.
We know Wasuki became Lynx because he was affected by the Frozen Flame when it was curing Serge, and then FATE took advantage of it, turning him into it's puppet and, after some time, into Lynx.
But, as far as we can see, the only change in Serge was he being cured, and turning him into the Arbiter.
There are many places where Serge is mentioned to "merge" with the Time Destroyer.
So here is where the teory takes place:
Not beign killed by Lynx, Serge would grow, and be more and more affected by the Frozen Flame. Sometime, he would either live A LOT or enter a freakingly long sleep, until the Day of Lavos. By that time, he would either be turned in a Lavos Spawn strong enough to destroy the world like the first Lavos was supposed to have, or merge with the dead body of Lavos (unwillingly, probably), bringing it back to life. Either way, the Chrono Trigger team would not expect a second Lavos, so they would not the there to fight him, and he would destroy the world without interferences.
This is why Serge was the one that HAD to save Schala, while anyone else could be the one Belthasar sent. By choosing to try to free Schala Serge avoided bringing lavos back, because: 1- He would die as once happenes, and Lavos would not be able to merge with him, or 2- He would use the Chrono Cross, and the music would ressonate and separate Schala and Lavos AND Serge and the Frozen Frame's power.
If the Chrono Cross is not used, nothing changes, and Serge becomes Lavos in Home world's future.
...A little too farfetched (is that the word?)?