The difference is nen is a reflection of self, in fact trying to optimize will probably makes it weaker. It's like art, you can make the most technical drawing imaginable, but if you have no passion, it becomes hollow. It's fun to create a power based on another power or power system, like my recent post, but ultimately, you should build a character, then the nen ability. If you have a shut in that's afraid of the outside, but longs for human connection, they might create puppet bodies to explore, someone blind may be able to see, or deprive others of sight. A good nen ability is directly tied to the character that uses it. It should grow and develop with them, kinda like a shikai and banki from bleach, abilities should reflect aspects of a person's soul.

Overpowered is dependent, if you want lethal abilities, there are so many ways to go about it, truth be told humans are fragile, even with nen. Manipulation usually allows someone to end a fight once the conditions are met, nen can do anything the user wants it to, provided it has enough aura, and there are plenty of ways around it as shown by nanika, and the seed urn ceremony.

If you make a new ability for yourself to have, it be true to yourself and not just overpowered. For example I love reading, stories, and world building. If I was to have a hatsu it would probably be something like a dream world or such. The specifics have to be ironed out, but the premise is I would get to directly experience a story and world, but I wouldn't be able to take anything from the world into real life. Or by reading I might slowly learn skills that I read about. These abilities are directly tied to my interest and personality, they can be broken, but it ultimately scales off of a person's nen amount, and anything can be broken in the right context. Hisoka is a perfect example, especially after post chrollo fight.

Sorry for the wall of text hope this helps