The phrase "inevitable sequel" means that a follow-up or continuation is bound to happen and cannot be avoided. It suggests that something, such as a sequel to a movie or a consequence of an action, is expected and will happen naturally.
The director was only given his relatively small $ 250,000 fee upfront, but baked in a part in his contract that adds $ 250,000 for the total fee for the now inevitable sequel. (inverse.com)
One in the middle hinting to a possible if not inevitable sequel. (fatmovieguy.com)
While this unwanted yet inevitable sequel probably still can't be considered a commercial success, worldwide returns make it impossible to call it a flop. (dvdizzy.com)