I'd suggest 'Ao Haru Ride'. It shares similar themes of young love and self-discovery like Fluttering Feelings, and the art style is also somewhat comparable.
Maybe 'Kimi ni Todoke' is a good match. Both have a gentle and heartwarming tone, and deal with the complexity of human emotions.
You might find 'Fullmetal Alchemist' somewhat similar. Both have deep storylines and explore themes of morality and human nature. Also, the art styles have certain similarities in their detailed and immersive depictions.
I think 'Berserk' could be similar. It has a dark and intense style, and complex themes like violence and fate.
I think 'Love Hina' could be similar. It has a similar lighthearted and romantic tone.
Perhaps 'Elfen Lied' would be of interest. It shares a similar level of intensity and emotional depth as 'Sick', with complex character developments and a unique art style.
I think 'Fruits Basket' could be similar. It has elements of emotion and relationships like in 'Happiness', with a charming art style.
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'.
One manga that's somewhat similar to Bleach is Naruto. Both have action-packed storylines and diverse characters with unique powers.
I think 'Love Hina' might be somewhat similar. It has elements of romance and character development like in Sextudy.