The idea of Eren Yeager in a harem scenario is interesting because it takes a character from a serious and often dark story and puts him in a more light - hearted, relationship - focused situation. It allows for different character interactions that we don't see in the original work.
There are various 'Eren Yeager x Harem' fanfictions. Some stories could be about Eren being in a harem situation where he has to balance his duties and his relationships. Maybe in one, he is training to fight Titans while also navigating his feelings for multiple girls. These fanfictions often play with the existing character dynamics and create new scenarios for the characters to interact in.
Mikasa and Eren don't kiss in the manga. The focus of their connection lies in different aspects of their shared experiences and the evolving plot rather than a romantic kiss.