Unfortunately, there's no choice but some sort of path to citizenship for the millions that are already here. The problem is too big. You can't go round them all up. What you can do, as an example, is seal the border first, then have them all register, give them a time frame (1 year or something), put them at the back of the line. In the mean time, they have to go through the same citizenship requirements everyone else has to, etc, etc. For the ones that fail to register, after whatever time frame, and they get caught (pulled over, etc) they get deported. Maybe give them extra time to register if they have a legal resident sponsor. Or something.
Point is, it's not perfect, but we are were we are. It's a path to citizenship, but not an amnesty at all. It's humane and fair.
Just an idea.