I actually really like this, and think Michael B. Jordan would be a really good fit in the role.
In Reloaded, Trinity tells Neo that Morpheus was with Niobe until Morpheus went to see the Oracle, at which point “everything changed” and Niobe ended up with Locke. There seems to be a lot of history between Morpheus, Niobe, and Locke…maybe the arc of the movie could follow Morpheus becoming unplugged, learning the truth, meeting and falling in love with Niobe…and then he learns the prophecy of The One, struggles between what he believes and his love for Niobe, and eventually chooses the prophecy over her…I don’t know, I’m just spitballing, but I do think that there’s some interesting stuff they could mine there if they wanted to.
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Plus, we could see Morpheus becoming distant from Niobe, the more he learns about Neo and the One. We would be able to see someone go from being a very emotionally connected man to someone that is emotionally disconnected as obsession begins to set in. And, perhaps, Niobe brings Morpheus to the Oracle hoping that she's able to talk some sense into him. The twist being that she not only doesn't talk sense into him, but only sends him further down the rabbit hole.
What would be cool about seeing this would be seeing Morpheus in Neo's shoes, hearing what he needs to hear from the Oracle.
I think Locke should also be involved, as you mentioned above.
For an example of a theme: Braveheart. That movie's theme is "freedom." Every scene in the movie is about freedom.
So whatever theme the Morpheus movie should have would basically help the movie as each scene would be about that. The original Matrix film was probably about "awakening." The movie even starts out with the line "Wake up, Neo." And every scene we see has Neo waking up in some way, to something he's never seen, thought, or knew before. I would think Morpheus could be about purpose. That seems to be what Morpheus was all about, unless you can think of another theme that would better serve his story.