According to game's director Tatsuya Nakae in a 2000 Arcadia magazine #3 developer interview, he was asked how Cable and Marrow were selected for this game? He responded:

"One of the staffers at our company is extremely well-versed in American comics, so we asked him to come up with a list of candidates. Using the selection criteria of “attacks that can be easily understood at a glance” and “doesn’t significantly overlap with any of the characters introduced with the previous games”, as well as “cool factor”, we narrowed the selection down to those two."

"With regards to Cable in particular, we also factored in his slightly convoluted backstory as Cyclops’ son from the future as something that players would find fun."
Contributed by ProtoSnake