Practically all US airlines, even those with no bargaining agent or working agreement, follow the seniority system. Even at the Regional Airlines when there is occasion to hire "off the street" captains (lack of FOs with quals or lack of bidders), those who were hired before will go above the OTS hires when they get the time/chance to upgrade, leaving the street hires on reserve semi-permanently until they too get seniority. The increase in pay those segments have seen in the last few years have essentially eliminated that anyway.
Unless you have a US ATP/rATP, no 121 flying at all. That means 1,500 hours (among other requirements) unless you came out of the military or a university, and even then the requirements are fairly high.
Unless you have 1,000 hours of 121(or very similar military time, but only credited up to 500 hours) time, no upgrade.
If you already have a Green Card (through the lottery or marriage), you can apply, but you'll start at the bottom like everyone else.
Work visas are so improbable, it's not even worth considering. As long as there are regional airlines to hire from, there is no need and the hassle for the carriers isn't worth it.
Australia gets a special deal because they have what some consider a "special relationship" with the US. Just like paying back a best friend, this is a perk for being there when they were/are needed.
There is no pilot shortage in North America, no matter how many people repeat it. The only "shortage" is a lack of applicants for operators with insufficient pay, benefits & work rules.
Never say never, but this will not likely change in any form for any reason in any sort of foreseeable future.