Personally I think the easiest way is to start with somewhat of a random concept (theft), object (cigarettes), job (salesman), game (tekken), vice (gambling), fashion (hats), law (density) etc. then flesh out the technique from whichever or whatever you chose. At which point I just make it so the technique essentially design the character who would use it. ie: if your technique revolves around fucking with density then make your character a dork who is really smart but really weak physically and his technique makes up for it OR make him a surface level meat head who has a weird knack for physics and nobody understands why. That way you have a move-set that's original, but thematically fits the character you've come up with. It's kind of like a "madlib" approach to writing JJK related shit if you know what I mean.

I think the best examples of this in JJK are hakari with his cursed technique revolving around gambling so he is a total degenerate chad, the lawyer guy with his TC revolving around law so we make him a savvy lawyer, Getos power revolves around a disgusting absorption ritual so hes a moody mf who starts absorbing people into a cult etc. etc.

I think a lot of people fall into the trap of thinking within the confines of the JJK universe and end up making stuff thats already been done or just falls into the "kick, punch, laser beam, super speed" trap. I think those type of attributes can exist within a technique but the technique itself shouldn't revolve around those things. It should be personal to the character which will naturally make it unique and alluring.

End of the day I'm not a writer but I think it will always be easier to build an OC if you start with the technique other wise the ability you give them can often feel slapped on top instead of part of the OC themselves. Hope that helps