It won't be politically possible to change much of anything about the legal immigration system until we stop the ~200K[1] of people illegally coming across the border every month.

The minute you announce some kind of concession for people already here (which would most definitely be part of a broader immigration reform bill) then the floodgates will open with people trying to make it in before the effective date.

[1] https://www.cbp.gov/newsroom/stats/southwest-land-border-enc...