It all depends how Guardiola deploys Messi, really. City's team is well drilled, they have clear patterns in the build up and organized runs in the attack. Messi hasn' had a team like that since at least 2015, probably even 2013 which has lots and lots to do with him dropping deeper and deeper in recent years. In Barcelona's prime years, Messi could focus on the final third and was incredibly hard to mark due to his false 9 movement. Wide forwards making deep runs and him dropping into the half spaces always ensured that somebody could find him with a pass between the lines. It'll be interesting to see if he can at least somewhat reproduce his output in such a team. City definitely don't need him to initiate attacks from deep like Barca did in the last years.
What's probably even more interesting is the question how Messi's integration would affect de Bruyne's playing style. Right now he's occupying lots of spaces Messi likes to operate in. And de Bruyne also has the privilege to lose lots and lots of balls. I think if Messi really joins City, Guardiola will ask de Bruyne to play less risky.
