Decide how you’re using piquillo: blended for sauce, chopped for topping, or layered in a dish.
Start with the 1:1 swap (roasted red bell peppers). For more piquillo-like roast depth, add a pinch of smoked paprika.
If your substitute is sharper, stir in a small pinch of sugar; if it tastes flat, add a few drops of sherry vinegar or another mild vinegar.
Match texture: drain excess liquid for chopped use, and blend slightly for a smoother, piquillo-like puree.
Taste and adjust in small increments (smoke → sweetness → tang) so you don’t overpower the dish.