In complete seriousness: you need to

A) give him a catchphrase

B) have someone use him for their own purposes and turn out to be a friend later on after saving him from a tough spot

C) make sure he has no luck with girls except herione and make her tsundere later on.

D) add more random fillers to ensure suspense and it also gives you some relaxing time+opportunities to add more characters.

E)amnesia arc: a slime man with amnesia, you gotta love it.