[SPOILERS]
This is the stage I set up in my mind for this confrontation:
Light has already defeated L, and he's on his way to create the new world in which he is the god. Lelouch won't accept Kira's world plans because it is a world ruled by fear (similar to Schneizel's plan with the FLEIJAs), so he creates the black knights to put a stop to his plans.
In this reality, the empire of Britannia doesn't exists, either Near nor Mello. The only master minds at play here are Light and Lelouch. Still, Lelouch keeps his title as former prince who is now thought to be dead (so he was a public figure, with the risk of being traced back). And Light has in his pocket the option of making the deal for the Shinigami eyes.
Who do you think would win? Is there enough space in the world for two gods?
Write down in the comments your opinions and reasoning.
I think there are many variables at play.
Light's main advantage I would say is that he doesn't need to be in the place to use his power (he could fight this entire battle hidden in an underground bunker with just a computer with internet access), contrary to Lelouch who must make eye contact to use his geass.
On the other hand, Lelouch has an limitless power to create an army of puppets under his control who could help him search the entire planet for Kira.
If I have to choose I'll say Light's biggest chance is to discover Lelouch Bi Britannia is alive and by connecting the dots determinate Lelouch is Zero.
Lelouch biggest weakness is that he once was a public figure so if Light allows himself to start killing any possible suspects he has the advantage.
Also, although the death note doesn't allows him to order someone to kill another person, if he finds the identity of one of Zero's followers, he could order him to remove by force Zero's mask in public (Zero tends to make many public appearances along the anime).
With that last option, what do you think would happen??? If Lelouch uses geass on someone ordering him to obey only his orders, but then Light writes in the Death note that that person does something and then dies. What would happen? That person would obey what's written in the death note, or would it be considered a "physically impossible action" according to the death note rules (because of the geass at work) and that person would just die from a heart attack???
Music of the Trailer: No Man's Land, by Revolt Production Music
Used in the Middle Earth: Shadow of War Story Trailer
I know that they say the word "world" 10 times throughout the trailer, but, what can I say? They both love to say it.