first thing the player's union would want is a higher cap or no cap.
Well no cap, it doesn't work. They always WANT a higher cap. But that's the point, within the same cap rules, it's no less money for them. Just means the top guys make less, and the bottom guys make more.
You could actually make it even better for them, and not make it a "cap" make it a team salary. So really, right now every team is some amount under that "cap" so that brings every team up to the line. I pulled up the Lions, just as an example. They are $7.3 million under the cap, even accounting for dead money, which would also no longer be a thing. You would also set a separate standard practice squad salary, that could be prorated based on time spent called up, on a full NFL salary.
So take the cap this year, $177.2 million. NFL has 53 man rosters, split that up evenly, that's roughly $3.34 million per player. That's right about on par with what the 12th highest paid player on the roster is making this season. That's the cap hit number, not the actual paid. Only 6 guys are actually making that money this year, the rest got it up front as signing bonuses.
So the issue isn't the amount, the amount is the same. Roughly 75% of the guys in the league will see a raise, many a substantial one. Hell, most will wind up making more this season than they would have for their whole careers. The problem on year to year deals like that would be injuries, and the lack of any sort of guaranteed deals, but most NFL contracts are only minimally guaranteed anyway. This may lead to quarterbacks having their own separate union of entitled princesses, but the the large majority of the union's membership, it would be a huge boon. I also think you'd see guys stick around longer because you wouldn't see teams opt for young guys, because if it's close to equal you'd rather take a guy on a rookie contract than pay a 28 year old the veterans minimum to be a backup, but if the pay is the same, that would actually probably reverse, and teams would hold onto guys for longer.