came back a few times

Yeah, I've heard that Sheridan was going to be introduced all along, and I think I might know how he would've fit in.

From JMS and "The Lurker's Guide:"

On "Babylon Squared"

Q: "So who IS the One? Some of the evidence points to Sinclair, but other bits seem to indicate Delenn. Yet neither seems to fit all the facts above."
Exactly.

JMS: What you have here in your message are two pieces of the puzzle. You're confounded by the fact that somehow they don't quite seem to fit into one another. That's because there's one last piece missing in this part of the picture, which fits in between them. The intent is to put this piece into clear view in year three, probably between episodes 8 and 11 approximately. At that point, the question of the One will be fully answered.

Now let's take that statement, and look at it.

Between eps. 8 and 11 of season three. That's really close to the time when Sheridan seceded Babylon 5 from the EA, and also close to the events of "War Without End," where the meaning of "The One" was explained with Sinclair being the One Who Was, Delenn the One Who Is, and Sheridan the One Who Will Be.

I have a feeling that had O'Hare stayed, then "Sheridan" might have been brought in in a capacity similar to General Hague, an advisor and member of the breakaway movement, until he would've been brought in as a regular during "Severed Dreams." As you recall, in that episode, General Hague was said to have been killed (actually, he was killed offscreen because the actor wasn't available due to a gig on DS9). Since Sheridan was already onboard B5, Hague would cease to be necessary at that point.

At which point, Sheridan would be there to take over B5 when Sinclair rode B4 into the past. Providing Sinclair was to have left the series at that point.