1)If your trying to make a weakness have it relate to the user's power for instance don't have a fire power where their weakness is they become dumber when they use their power instead make it so their insides burn if they overuse their power or something heat related

2)Quirks or any other power don't have to powers only used for combat a user's power can be used to heal, sneak past people, or support others it doesn't need to be a flashy quirk like Bakugou or something

3)Try to get inspiration from other shows or media but don't copy off of them. For instance if you want to make a quirk that's based on let's say Venom you could make a fun power that allows the user to have more then one power but it shouldn't be in the form of black suit that looks like Spider-Man's costume(Honestly Ive slightly betrayed this rule on a few quirks but my point still stands)