Well, I'm not Kevin Feige, who could provide a definitive understanding of what a standalone movie mean to Disney/Marvel, but I do not see that happening with that particular character.
If the MCU future plan is the same as in Phases 1 through 3, every movie is a sub-plot within an all-comprehensive plot. All movies have some large or minor tie with an upcoming film. Like Phase 1, Phase 3 is a restart of the new overall plot (whatever it may be).
Phase 1 provided examples of minor ties involving ending credits cut scenes:
- Iron Man - Director Nick Fury telling Tony Stark about the Avengers Initiative (The Avengers)
- The Incredible Hulk - (immediately before ending credits) Tony Stark tells General Ross "they" are pulling together a team (The Avengers)
- Iron Man 2 - Agent Coulson finding Mjolnir in the desert (Thor)
- Thor - Fury showing the Tesseract to Professor Slevig, with unseen Loki using his shadow magic to watch and manipulate Slevig (The Avengers)
- Captain America: The First Avenger - Basically a The Avengers trailer
Perhaps a minor tie-in, included to a previous movie, would be the best prelude to a standalone Cable movie based upon what has been seen so far in the MCU. Then the question becomes what other movie (or television series) would include the tie-in? Coming up are:
Black Panther: Wakanda Forever
Ant-Man and the Wasp: Quantumania
Guardians of the Galaxy Vol. 3
The Marvels
Fantastic Four
Blade
She-Hulk
What...If? season 2
The Guardians of the Galaxy Holiday Special
Secret Invasion
Agatha: House of Harkness
Armor Wars
Echo
Ironheart
That may be an incomplete list of confirmed films and television series. Which movie or series could have a hypothetical tie-in to a
time traveling, telepathic telekinetic, firearms master? I have a crazy imagination...
Guardians 3? Nathan might sense the threat of Adam Warlock and try to stop it but arrives after the team wins.
Fantastic Four? The team battles Kang the Conqueror, who opens a portal escape to Nathan's time.
Secret Invasion? Nathan gets mysteriously sucked in at the conclusion of the conflict, much to the surprise of Nick Fury. It's "secret" so anything can happen.
Now a realistic guess for me would be an appearance in
What...If? The series, like the book, can explore anything imaginable or unimaginable. Like Captain Britian, they can shelve Nathan's eventual movie appearance when the time is "right".