I'd be (pleasantly) surprised; look at how long it takes them to allow system apps to get replaced with downloadable 3rd party.
And even then, the activation keyword is likely to be whatever Apple says. Similar logic as 3rd party keyboards, don't want user input to get stuck on even merely potentially untrustworthy or buggy code.
Correct me if I’m wrong but isn’t the activation word “special” in that it has to work in very low power states? I always assumed that is why the wake words are fixed, because said words need to “fit” within a very small power budget.
And even then, the activation keyword is likely to be whatever Apple says. Similar logic as 3rd party keyboards, don't want user input to get stuck on even merely potentially untrustworthy or buggy code.