Bit difficult on the worldbuilding side, if we want to make them happen in the same earth, but there's always the multiverse or other shanenigans.
See, my thinking was we merge all 3 settings into one in some kind of Event in much the same way that the company
Wizards of the Coast did for Magic:The Gathering and the Mending.
In fact, I think we have an interesting starting point in The Secret Saturdays. Canonical, Zack's parents were dubbed Secret Scientists, and we know
50 were active globally and participated on the raid against Argost's base. Of this
50,' More than half of them were lost.' (The lackluster wiki). We also only ever see 7 Active agents, but
43 casualties seems kind of insane. That means 40+ people with the intelligence, drive, R and D capabilities, or experience with the esoteric or Strange are no longer pushing the bounds of Human progress in this setting. Or more concerning, that's 40 less Geniuses monitoring the world for
Anomalous or Criminal Activity. What if there's some event or series of events going on that would have been watched, but now,
Nobody knows is happening? We can have runaway events or cascading situation which merge and rewrites multiple realities. The movies then would be different characters engaged in their routines or resolving other problems. They could be engaged in foiling villainous plots, exploring their respective worlds, or engaged in their daily routines…..
Only, some people have noticed peculiarities. Maybe, the security guards in the Null Void notice this dimension space is been
strange for the past couple of days. Maybe, Providence of Generator Rex discovers ancient Ruins without realizing these Ruins were never native
to their Earth. Or maybe, some characters don't even realize anything strange is going on because they're
in the eye of the hurricane. However, nobody realizes they have bits and pieces of evidence to a wider event, or if they do, then they're not in a position to change what's about to happen. This Confluence is a Tidal Wave, and like driftwood, everyone else is along for the ride. Thus, is the Ben 10, Secret Saturday and Generator Rex Cinematic Universe is born, and these the stories about what happened Before, During and After.
Taking a step back, we can introduce the works or other Secret Scientists that could have found hints to Cosmic Events/ Issues with Reality which could have foretold/discovered evidence for the incoming merger. Ep 1 of the original Secret Saturday's cartoon has half+ scientists gone, so with half as our starting point, let's say 50 Scientists ÷ 2 = 25 Scientists.
From this 25 Scientists - 7 [Canonically Active] = 18 Scientists, but there are fates worse than death. Moreover, people can choose to not be scientists anymore. Maybe, they got crippled from Argost's mansion. Maybe, the trauma of what they saw convinced them to renounce and give up scientific endeavors. Maybe, they choose suicide in the aftermath of all that loss.
Regardless, even cuting our pool of 18 Scientists by half means we have 9 potential OCs to create and weave together a wider plot. Heck, we can even say the Scientists who
might have found evidence perished in that Mansion before they could tell anyone, leave notes, or discussed with their peers.
secretsaturdays.fandom.com