Yes, it is. As a 'Code Geass' fan, it gives you more content related to the series. You get to explore the story further and might discover new aspects of the characters and the world.
In a crossover, the two different power systems would create a really interesting dynamic. The Death Note's power to kill anyone whose name is written in it, and the Geass which can control others' actions. If Light and Lelouch were to cross paths, there would be a battle of wits as they both are highly intelligent characters. Light would be cautious of Lelouch's Geass, and Lelouch would be wary of the Death Note's invisible and deadly power.
The unexpectedness. Since Lelouch is such a central and powerful character, his death can completely change the dynamics of the story. It forces the writer to explore new storylines and character developments.
It's hard to say who would be the most powerful. Light has the power of the Death Note which can kill anyone directly. However, Lelouch has the Geass which can be used in very strategic ways to manipulate people. In a direct confrontation, if Light knew Lelouch's real name, he could write it in the Death Note. But if Lelouch could use his Geass on Light before that, he could make Light do things against his will.
Code Geass did start as a manga. However, its anime version became even more widely known and beloved. The manga provided the foundation for the rich story and characters that were further developed in the anime.
Code Geass started as an anime. The manga was based on the anime's success and came after to expand the story for fans. Some elements might differ between the two, but the anime was the original form.