It has to be some measure of how much I enjoyed the film that I'm actually excited at the prospect of Cable appearing in the sequel. Assuming that's actually something that's going to happen, and given there are actors actively campaigning for the role, I guess it is.
Admittedly, I only enjoy Cable when he's around Deadpool, but half the time I'm infuriated at his vaguely patronizing attitude towards Wade. But I'm curious to see what they'd do with him, how they'd get him into the movie in the first place. Will he already be in the present? Will he time travel in partway through? Will Wade travel into the future? How happy will Wade be to play with future weapons, like laser rifles that don't require ammo (that he would just forget anyway)?
I was wondering how they were going to build Wade and Nathan's peculiar friendship in one movie, considering how long those two were full-on antagonists, then grudging allies, then actual buddies and whatever else they might be. But movies pull off that antagonists to buddies stuff all the time, like Lethal Weapon, or Tommy Boy, so it's easily doable. Although Deadpool/Cable kind of runs a wider gamut, since they were actively trying to kill each other, and I'm not going to rule out them being presented as a couple in the film (certainly expecting Wade to make several suggestive remarks).
I'm assuming the X-Men defeated Apocalypse in the recently released X-Men film*, which is a bit of a shame, since he's frequently the reason Cable's running around in the present anyway. But I suppose it's always possible that Apocalypse didn't actually die**, and he's back causing trouble, and that's why Cable showed up, to finish the job.
* Which I might get around to seeing one of these days. I haven't really loved any of the X-Men films, and I still haven't seen Captain America: Civil War, even though I did love each of the first two Cap movies, so it isn't exactly a priority.
** I've come to expect superhero movies to kill their bad guys. I don't really like that they do it, but it does happen a lot. But maybe Apocalypse didn't die, even though Jean incinerated his physical and astral form, thank you Wikipedia.