WHC: Create the most prosperous Constatinople as realitically possible

Its quite a stretch to expect the Byzantine empire to last longer than it did. With Ottoman expansion, the writing was on the wall and the empire was always crippling itself away. That being said, there were many key moments in history where Byzantine could've stayed stronger and been more stable by the time of Ottoman expansion.

Two obvious ones that come to mind, is an avoidance of the people's crusade and the fourth Crusade. I don't think its possible to last longer than 1500, but Constantinople as a city could've been much better positioned by the time of Ottoman conquest.

So my question is; what are some events that could have greatly empowered Constantinople (both the city and the state) by the time the Ottomans arrived?
With no Fourth Crusade, a Roman state could easily still exist to this day. Even with it, I don't think it crossed the point of no return until the civil wars of the next century.