Death Note
This is the most famous pairing because both follow a brilliant antihero using a supernatural power while hiding behind a double life. Fans who liked Lelouch's manipulation, moral compromises, and battle of wits usually get the same rush from Light versus L.
Guilty Crown
Guilty Crown is often recommended because it has rebellion politics, special powers, dramatic betrayals, and a stylish school-to-war escalation. It is less tightly written than Code Geass, but the visual melodrama and resistance-against-an-empire setup overlap strongly.
Legend of the Galactic Heroes
This is the bigger, more serious political-strategy recommendation. It shares Code Geass's interest in charismatic leaders, ideology, military tactics, and the cost of changing history, but with a much more grounded space-opera tone.
Mobile Suit Gundam 00
Gundam 00 shares mecha warfare, global politics, terrorism/resistance framing, and pilots caught between ideology and personal loyalty. It is more ensemble-driven, but fans of Code Geass's political mecha angle often see the connection.
Aldnoah.Zero
Aldnoah.Zero gets linked because of its tactical mecha battles, war between political factions, and brain-over-brawn confrontations. It is more focused on battlefield problem-solving than Lelouch-style revolution, but the military sci-fi tone is close enough for many fans.