I would make a law where anyone can come over to the United States and stay, regardless of their legal status. Open borders, where anyone, from anywhere, can, in exchange for providing ID, a customs/smuggling/invasive species check of their belongings, and maybe a health check, they get in. Once in, a separate but similarly simple process to get a green card. Following a suitable period of residency, say, 5-10 years, citezinship (if they want it). It would be suitable for this law to fall under the Legislative Branch, as it makes all laws (including laws relating to immigration), declares war, regulates interstate and foerign commerce, and controls taxing and spending policies.