Where Alec is an angel and Magnus is a demon. Sometimes heaven commands Alec to chase Magnus for his sins on earth. Other times, Magnus tries to tempt Alec or they just have a forbidden love. Every time they fight against heaven and hell to be together. Possibly they also try to stop the apocalypse. Hallelujah!
Also, this trope could include all the fics where Alec has wings, Magnus is more demonic or inherits Edom.
Where Alec and Magnus are soulmates and share soulmarks or their marks complement each other. Alec usually hides his mark because he’s embarrassed to be tied up with a downworlder and the same is true for Magnus who is bonded with a shadowhunter. But when they finally meet, they connect and, with a little effort, they could both break their own biases and be happy.
𝙿𝚘𝚙𝚞𝚕𝚊𝚛 𝙼𝚊𝚕𝚎𝚌 𝙵𝚒𝚌 𝚃𝚛𝚘𝚙𝚎𝚜 (8/?):Reverse 𝙰𝚄
Where Alec is a warlock and Magnus is a shadowhunter. The character roles are all inverted (shadowhunters are warlocks and vise versa) but storyline follows canon. They conserve their original personalities so Alec is a grumpy hermit and Magnus loves flirting and fashion.