Why can’t an atom gaining protons just remain the same element?

Because the number of protons defines the atomic number of the element, adding a new proton will not only change the number of protons but also the atomic number. The atomic number is how an element is determined, so changing it will change the element

plasma is the state of matter where the substance is ionized and has no definite shape or volume. the fact of being ionized makes the substance a good electrical conductor.