Immigration process reform for one. Revamping how we deal with illegal immigrants rather than shipping them into ICE facilities for years. Best sources of statistics state that between 31% and 54% of illegal immigrants were originally in country legally to begin with. (Either by visa, or some other means). So 1/3 to 1/2 of the issue is completely unaffected by a wall from the beginning.
[See:
http://www.gao.gov/new.items/d0482.pdf,%20https://www.dhs.gov/.../publications/Ill_Report_1211.pdf,%20and%20http://www.politifact.com/.../rubio-says-40-percent.../]
The fact remains we have what appears to be a border fence through most of the easy-to-install areas. The bulk of the unsecured US/Mexico boarder is on the Rio Grande and for obvious reasons. The river is not straight, the ground is not easy to build on, and we have US citizens that live and depend on services on the river itself. So access through a wall to get to a home, or business needs to be provided.
Add in the fact that eminent domain is often abused.
Reform the processes that are failing. I am not saying it's easy. But building a wall serves what purpose exactly? It does not stop someone coming legally and overstaying their welcome. It won't stop the criminal element from using their resources to continue various criminal activity. At best it is a boondoggle designed to act as theater to give the American people an illusion of safety, much like the TSA in airports.
My ultimate point is this. No facts I have ever seen have proven the current fences and walls provide adequate security (and we have miles and miles of them). They are circumvented via tunnels, ladders, tools, and creative thinking. How does building a wall stop this? Why waste money on something that does not do anything?
Keep in mind I am not a Democrat. I am just against wasting good tax payer dollars on government programs that just grow without thought.