I seriously dislike any theory that says that Lavos will move on to another planet, since the Lavos Spawn are supposed to be his legacy. It'd be totally stupid to have children if he's gonna persevere.
Well, working with that idea, it could just be a little twist on the whole idea of the DNA perfection dream of Lavos.
Why do people have children? Baseline, it's written into us. Natural instinct to continue on our species. But that's just animal, we're (not too be too egotistical about our species n' all) above that. Humans reproduce for a deeper reason then to just preserve humanity, we do it for immortality. We truly and deeply wish to continue on our names and memories, our experiences and wisdom.
In a similar way Lavos is immortal by continuing to reproduce, passing on the DNA collected from a planet to his offspring so they can in turn find other planets, collect their DNA, and grow ever closer to biological perfection. That falls apart though with the lack of exchange in the species. Lavos don't mate, they just sap energy then reproduce... outa' somewhere (Don't get into -that-). So I think only one Lavos makes it into space (the others die off, possibly some battle of dominance, I'm not sure), a single creature overflowing with the DNA information and advantages of all his kind before him and infects a new world.
This does seem to fit pretty well with most of the Lavos theories thus far, but it's more in support of the idea that Lavos, as a species, has this dream of perfection (Like teh BORG :3 I'll shut up) by collecting the DNA of species throughout the endless expanse that is the universe. It's just that dream is passed down through the generations of lil' Lavoses... maybe?
Note: Yeah you people have jumped into an ethical question of 1 life versus a billion. Sorry. My two cents? It's wrong just to start. Why not two? Whats two lives next to a billion? How about four? Still small. Ten? Twenty? A hundred? A thousand? A million? Still small next to a billion lives. It's far too easy to play god.