Two thoughts ... in PRO football -- Bulldogs is one that could used and embraced. I'm actually surprised it's not used. Of course in college and HS that's another story. But, I'd love a Bulldogs name.
I'm sure you're right about Athletics, Packers and Magic. But, it's also hard to compare the three since they all come from different eras and got their nicknames through different methods (nickname from newspapers, sponsor, name-the-team with heavy marketing considering) ... but, now, in today's American sports world we've kind of established what kind of nicknames work for professional sports -- animals (Lions, Tigers, Rams, Hornets, Ravens, etc), regional names (Nationals, Texans, Canadiens, Phillies, Yankees), regional appropriate names (Lakers, Jazz, Oilers, Nuggets, Spurs, Magic, Hurricanes, Canucks), mythical names (Wizards, Titans, Knights), people's nicknames (Redskins, Blackhawks, Celtics). I'm sure I could make that list longer especially considering early 1900 baseball nicknames.
There are other "rules" we've kind of adopted ... singular nicknames are not always the best (Magic, Jazz, Wild), and double name nicknames are pretty ridiculous, especially for top tier professional sports (Mighty Ducks, Golden Knights, Silver Stars, etc). Though those rules are thrown out the window for minor league (especially baseball).
My point being -- if the XFL wants to be taken more seriously I'd avoid minor league sounding names. They can get creative, just don't go overboard IMO.