I can think of many bands they could open for.
Alter Bridge is one.
Yeah, I can see that working, but Alter Bridge doesn't really play the bigger rooms that modern bands like Breaking Benjamin, Disturbed, Halestorm , etc. do for their own (single headliner) shows. They only play up to like 1,500 people at a time in North America. Europe is a different story. Also, a reference point here is a few bands that opened for AB in the last few years in the states (some of you guys probably don't know these bands): Monster Truck, Adelitas' Way, Nonpoint, All That Remains, and soon to be Clint Lowery of Sevendust.
I don't think it would benefit all that much Sons of Apollo playing venues that holds 1,500 people in comparison to what they were playing in their run in 2018. Getting festival spots in Europe in festivals like Download, Rock AM Ring, Graspop, Hellfest, etc. would benefit them big though.