The only way an XFL type league will work is if it is NFL supported (see G League) and teams are actual farm systems of some sort.
Let NFL practice squad players play in XFL.
Biggest thing is LET NFL 3rd string QBs (they'd have to define this very specifically but perhaps each team could block only 2 QBs) play in XFL.
QB play is issue with lesser leagues.
Ive always agree with this as well,, would make the NFL a much deeper league from top to bottom.
A lot of young players get old on the shelf quickly in the NFL, only so many you can dress on gameday.
Having a farm system of maybe 15 players, there doesn't even need to be a game structure, just an expanded PS,, but leave the PS,,
add an expansion of 15, they can get plenty of simulation reps, mastering the playbook, scheme, main thing is they are active,, when you want to call one up, he replaces one of your PS players, and that PS player is no longer retained,, FA PS guy...lol
There would have to be certain margins in place,,, Like you couldn't just have 15 QBs