Gimmicks. They show up. They always do. Everything is sorta technically a gimmick anyhow. Weird word with a lot of scope issues. But it's here to stay, and so are gimmicks. In the name of flavor and fun, I want to empower you to take a free starting gimmick for your character just as you would (read: will) take a free starting feat.
The primary function of a gimmick should be, well, none. As I'm defining them for this house rule, a gimmick should be a piece of lighthearted and perhaps somehow unexpected flavor for your character that is mechanically impotent despite the positive effect I hope it will have in lubricating rp and boosting session mood when we need to.
Of course, anything can be mechanically very powerful in the right quantity. Every gimmick worth its salt could be abused for mechanical benefits. But let's please not do that. I'll provide a possible example. Sinn is a soul knife rogue whose gimmick is that he always has a pitcher of lemonade or iced tea on him. If he offers you lemonade at an emotional time and you throw the pitcher in frustration, he is welcome to pull out one of his improbably many backup pitchers for the sake of the gimmick. I think it's quirky and fun. But I don't even need to type out the list of abuse cases in my mind for you tabletop deviants to know exactly just what sort of mechanical mayhem can be caused if a rogue is given a practically infinite supply of acidic beverage for free everywhere and all the time.
Personally, I think I'd like to let Willow Pill give me the fashion advice I need to sum up this house rule. A good gimmick is cute and absurd with a touch of ugly. By nature, they are exploitable like literally anything else. But so long as we agree to cooperate at our tabletop, I should like to invite you all to cooperate in having fun with me by way of character gimmicks.