Later this year, AMC will be the home of the next DC Comics adaptation, Preacher. Preacher will star Dominic Cooper as Jesse Custer, who along with his ex-wife, Tulip, and the vampire Cassidy, will be on the search to literally find God. Lucifer stars Tom Ellis as Lucifer Morningstar, the Lord of Hell who got bored of his life and left for Los Angeles, where he helps the LAPD solve crime.
Both of these shows are similar to the short-lived Constantine, with the demons, Hell and Heaven, and the Devil and God. The easier crossover, though, would probably be Lucifer, because that series has already started, and it's a lot more like Constantine than Preacher is.
Have John Constantine, Zed Martin, and Chas Chandler, the three stars of Constantine, all go to LA to track down a demon they've been hunting, and they must get the help of Lucifer and Chloe Decker to send the demon back to Hell. John would probably hold a grudge against Lucifer, because well, he's the Devil. But it could be explained by them making a deal, because without the two working together, the demon can't be sent back, or something like to that. The crossover pretty much explains itself, and I don't think it would be impossible to pull off.
Preacher would be the harder show to crossover, and I don't exactly know how you would do it. but I still think it's an interesting idea. Maybe Constantine, Zed, and Chas must go to Texas to send a demon back to Hell, but get the help from Jesse, Tulip, and Cassidy, while they are on their search to find God. That's the only way I can explain it, but I think it would be a great crossover, especially since, Constantine was awesome, and Preacher looks great!
Well, there you have it. That's how Constantine can continue: through crossovers. Crossovers with the two shows just like it: Lucifer and Preacher. This would also mean that both of these shows would be apart of the Arrowverse, existing alongside Green Arrow, the Flash, and Rip Hunter.
And so it's out there, I actually put these shows in the Arrowverse already. It would just be nice for them to officially be together. I also talked about the Dark Knight Trilogy and Green Lantern being apart of the Arrowverse right here: Why The 'Dark Knight Trilogy' And 'Green Lantern' Both Fit In The Arrowverse!
Also, just so it's out there as well, here is my Arrowverse, and everything I include in it, all in order as well.
1. Batman Begins
2. The Dark Knight
3. Arrow (season 1)
4. The Dark Knight Rises
5. Arrow (season 2)
6. The Flash (season 1)
7. Arrow (season 3)
8. Constantine (season 1)
9. Green Lantern
10. Vixen (season 1)
11. The Flash (season 2)
12. Arrow (season 4)
13. Supergirl (season 1, Earth-3)
14. Legends of Tomorrow (season 1)
15. Lucifer (season 1)
16. Preacher (season 1)