Just some crazy overlord crossover idea:

1:Gate - Jietai Kare no Chi nite, Kaku Tatakeri, and Overlord/オーバーロード

A mysterious Gate appears in the Sorcerer Kingdom of Nazarick and unknown soldiers attacking the innocent plebeian of Ainz. Now our beloved papa bones can't let others terrorize his kingdom population, do he?

2: Doom(2016) and Overlord/オーバーロード

After the ending of the Doom(2016) instead of going back to hell, the Doomslayer transported into the new world in the time of the evil deities.

Who will be stronger? The Slayer of Hell or the Demon-gods.

The Slayer can rip and shred his opponents as usual or he will be the one who ripped and shredded this time to pieces.

3. Game of Thrones and Overlord/オーバーロード

Instead of the New World, The Great Underground Tomb of Nazarick appears in Westeros. How will the rise of a new power will mess with the balance of power and the fate of Westeros.

4. Warcraft and Overlord/オーバーロード

Instead of Nazarick wholly transporting to the new world just Momonga or an another guild member or OC or a group of players transported into the Warcraft verse. Will they bring peace or total destruction? Who knows?

5. Bleach and Overlord/オーバーロード

Instead of the new world, Momonga will be transported into Hueco Mondo. Will he conquer this wasteland or going into adventuring as he always wanted? Or something else? What will the inhabitants of Hueco Mondo and the other powers say of his sudden appearance?

6. Dragon Ball and Overlord/オーバーロード

After a long night work Herohero going to sleep. After his waking, he found himself with his Avatar body and powers and in a world what seemingly "the not polluted" version of his own Earth. There are crazy things here. Flying humans and bugs whose can pulverize a mountain with one hit? What the…

7.Naruto and Overlord/オーバーロード

A human looking player character appears in the world of Naruto. How will he or she or it change the turn of events? Will the player will be capable to stop the Akatsuki to release Kaguya from her orbital prison or our player will be participating Black Zetsu evil plan to release his mother from her captivity and imprison everybody with The Infinite Tsukuyomi.

8. Prototype and Overlord/オーバーロード

After saving New York from the nuclear explosion Alex Mercer awakens in the New World.

Will he will be able to learn from his past mistakes and earn forgiveness. Or he will continue his path of destruction?

9. Mass Effect and Overlord/オーバーロード

After Ainz conquered the new world he turns his attention to the stars. After a long preparation, they will be able to advance that far to build starships and colonize planets.

How will Ainz and the Sorcerer Kingdom fare in the galactic community? How will the galactic community react to the magic?

Who will be stronger? The might of the Great Underground Tomb of Nazarick or the Reapers?

10. Dragon age Overlord/オーバーロード

What if Nazarick teleported to Thedas instead of the New World. How will Ainz and Nazarick will fare against Thedas? How will be the inhabitants and the chantry reacts to the beings who completely defy the Chantry their magic system and everything else what they thought?

How will the denizens of Nazarick react when the Inquisition trying to enforce their laws to Nazarick?

Who will be a bigger scourge in the pages of history?

Nazarick or the Darkspawns?

11. Hellsing Ultimate and Overlord/オーバーロード

A player appears in the middle of the night in Alucard little island. How will the Hellsing organization and Alucard fare against the player? He/she or it will join forces with Hellsing to stop a bigger threat? Or our player will try to conquer the world?

12. Soul Eater and Overlord/オーバーロード

Excalibur the Holy Sword transported into the New World. How will everyone react his appearance? He will manage to cause meltdown to Ainz at last or he and the denizens of Nazarick can achieve to make him stop his nonsense babbling? "FOOLS!"

If you have any other ideas wrote into the comment section. :)