Constantine was a decent enough film. But fans wanted it to be faithful to the comics and got something different. They got their wish with Matt Ryan, but even his tv series was lacking a bit. But both were enjoyable to varying degrees.
Marvel wants Keanu. They've been trying to get him for years and have offered him various roles (allegedly). I wouldn't say they were desperate. They are just very enthusiastic. I have no doubt they'll get him for something. I love Moon Knight to death, but the character just isn't big enough.
Another actor they really, really want is Joaquin Phoenix. Which is going to be impossible since he isn't "A franchise guy" (Ryan Gosling is the same). If it was a one off role? Possibly. But a role requiring multiple films and/or tv series? Unlikely. Which is why I am curious to see how they'll get him to do Joker 2.
Interesting times.