You might want to check out 'Scum's Wish'. It explores complex and unconventional romantic dynamics similar to what you'd find in 'Illicit Love'. There are elements of secret desires and forbidden attractions.
Well, 'Kiss x Sis' has some similar themes of forbidden love. The relationships in it can be compared to the intensity in 'Illicit Love'.
Often, themes like forbidden passion, social taboos, and the struggle between love and morality are explored.
I'd suggest 'Clannad'. It shares elements of heartfelt storytelling and explores various emotions, similar to what you find in 'The Promise'. There are touching moments and character arcs that might strike a chord if you enjoyed 'The Promise'.
You could check out 'Sword Art Online'. Both it and 'Gamer' have elements of gaming, adventure, and character growth in a virtual setting. However, they also have their own unique twists and differences.
One manga that could be similar is 'Elfen Lied'. It has elements of dark fantasy and complex character relationships like Narutaru.
One manga that's somewhat similar is 'Monster'. It has complex characters and a deep, twisting plot like Death Note.
One manga that could be similar is 'Battle Royale'. It features a group of people forced to fight to the death, similar to the survival aspect in Hunger Games.
Perhaps 'The Promised Neverland' is somewhat similar. It has a group of characters trying to escape a dangerous and controlled environment, with lots of mystery and strategic thinking involved, similar to the challenges in Squid Game.
I think 'Your Lie in April' could be similar. It has a grounded and emotional story like 'Down to Earth'.
I think 'Your Lie in April' could be similar. It has elements of complex emotions and explores the depth of human relationships like Love or Hate.
It's got a unique plot that keeps you hooked. The forbidden aspect adds a lot of drama and excitement.