Manny Coto and others involved in the production of Enterprise have mentioned that they planned to make Shran a member of the bridge crew in season five. Their motives are pretty obvious: Make a popular guest star (and the excellent actor who played him) as involved as possible. In Coto’s own words, “get Jeffrey [Combs] Somehow.”
The question for me is, why would Shran have been willing to take up a (presumably subservient) position aboard Archer’s ship? Shran is frequently shown in command of Andorian Imperial Guard starships, which gives him significant practical and political power. He is clearly a man of action, and strongly dislikes feeling indebted to anyone. He certainly does not strike me as someone inclined to surrender a command position.
So, how could Shran’s intended presence on the bridge of the NX-01 be explained?
In Shran’s penultimate appearance (The Aenar), the ship he’s commanding is destroyed, and he states he’s probably not going to be given command of another one any time soon. I assume this was meant to lead to him winding up on the Enterprise in season 5 if the events in the finale hadn’t happened instead.