One similar anime could be 'Sword Art Online'. Both have elements of fantasy, adventure, and characters with powerful abilities.
I'd recommend 'Re:Zero - Starting Life in Another World'. It shares some similarities with Overlord in terms of the protagonist's journey and the fantasy world-building. Also, both have moments of tension and unexpected plot twists.
I think 'Sword Art Online' could be similar. It has a fantasy setting and focuses on characters' adventures in a virtual world, like Overlord.
I think 'Cowboy Bebop' could be similar. It has a similar space adventure theme and a mix of action and drama like Firefly.
One that comes to mind is 'Fullmetal Alchemist'. It has a dark and intense theme, along with complex characters and a compelling story, similar to Attack on Titan.
One manga that could be similar is 'Watchmen'. It shares elements of dark themes and complex characters like Batman.
One manga that might be similar is 'Parasyte'. It has elements of horror and action, like Gantz, and explores themes of survival and humanity's response to strange threats.
I think 'Fruits Basket' could be similar. It has a similar emotional depth and character-driven storylines like 'Swan'.
I think 'Love Hina' might be somewhat similar. It has elements of romance and character development like in Sextudy.
I think 'School Days' could be somewhat similar. It has similar elements of complex relationships and emotional turmoil.
Well, 'Parasyte' could be a manga similar to Kaijii. Both have elements of horror and mystery, and the character development is quite engaging in both.
It's hard to say exactly. But you might want to check out manga like 'Attack on Titan' as it shares some similarities in terms of intense storylines and complex characters.