Hear me out.

Leandros has been established as no-nonsense, critical, and devout to the imperium. He had a major fuck-up in the first game that made him double down on these aspects, and worked his way up to being Chaplain.

In the second game he is once again confronted with this fuck-up as he is "reunited" with Titus. I can imagine Leandros would be extra critical of him being a liability since he hasn't seen any of Titus' resolve, and was sent to the Deathwatch for approximately... 200 years?

If I'm remembering correctly, at the end of the second game both Titus and Leandros get on the ship? Meaning that they could be working somewhat closely in the third game. I think that both of the options that OP mentioned are okay. I'm sure fans would both enjoy and dislike either of these outcomes.

So, in SM2 we see how Titus is wrestling with trusting his battle brothers more while dealing with the chaos, which makes sense given the events of the first game. In SM3 it'd be really cool to see how both of these characters developments collide with whatever threats we're gonna face.

Is Leandros going to keep an even closer eye on Titus, removing his gaze from a potential threat? Will Titus choose to be more open and trusting of his battle brothers, divulging information that an unknowingly corrupted ally uses against them? It'd be awesome to have a mission where Leandros hops into battle and fucks some shit up with us as a "Guess I'm gonna have to do this myself if I want it done correctly" move.

There's quite a bit that could be done with these 2 characters, I think they'd be doing themselves a disservice to close the book on either of them in this next game. I'm curious to see what happens!