With Deadpool 2 introducing the potential for Wade and Vanessa to start a family, it does feel like the wrong time to spring a surprise revelation.
It kind of all ties back to the reason why writers/producers Rhett Reese and Paul Wernick didn’t want to include the character detail in the first place, fearing that it would ultimately be too confusing for audiences.
In the comics, Copycat is a mutant who tried to infiltrate the X-Force by kidnapping Domino’s and stealing her identity, and introducing such elements to the live-action world would set up some interesting challenges for a couple that’s pretty much settled down.