As a historian of Turkey (though admittedly not one with a specialty in Ottoman history) I would love to add my five cents into the discussion.
1) Persia must be re-worked/buffed as necessary to make it a formidable rival to the Ottoman AI. Otto-Habsburg rivalry most certainly defined the history of this part of the world for a while, but Otto-Persian and eventually Otto-Mughal rivalry too (yes, latter even worked with the Portuguese against Ottoman ambitions in the Indian Ocean) must counter Ottoman ambitions in the East. A strong Persia with diplomatic ties to Western powers can be a great check for the Ottoman AI whilst making the game somewhat enjoyable and interesting for players who wish to play as the most OP state in the game. This really must be the priority.
2) As a lover of smaller states, I would love to see new content for Caucasus and Arabia regions. Caucasus must be made less palatable for wannabe Empires that wish to conquer it, and Georgia as well as Armenia should receive extensive and interesting missions that are tailored for human players. So in theory, they should not become OP on their own when AI plays them, but for skilled players a path to recreating the ancient Armenian Empire or forging a mighty and tall (production based) Georgian Caucasus Empire should be possible.
3) I prefer quality over quantity and the quality of work done by Paradox in the latest DLC was just astonishing. Similarly, I think it is best to pick 5-6 states to really re-work and come up with a decent mission tree etc. for rather than making smaller and not so fun mission trees for 10-20 states. Ottomans, Ajam, Mamluks MUST receive a re-work to their missions, I would like to think that this is non-negotiable... On top of these, Georgia and Karabakh/Armenia can receive a mission tree too, that would be most interesting for a lot of players. An extra one or two mission trees if possible could be made for a selection of these few states: Byzantium (community favourite, would not be my pick), Knights/Cyprus (Gotland-like option, would be amazing), Genoa/Venice (unlikely but they too can bring flavor to the region), Crimea (my favourite option).
4) Last but certainly not least, manpower/force limit rework is needed for the Ottomans. The current system makes the state OP through sheer numbers and some modifiers, and makes a very poor case of showing Ottoman military might. It is, basically put, just any other "Eastern horde" with a flood of troops and nothing else. The reality was quite different. While the earliest Ottoman conquest were often full of Christian volunteers who wanted some cash, later campaigns in the Balkans would be fought with numerically not so superior but very well equipped and trained Janissaries. Ottoman population too was relatively small compared to some of its European rivals, almost dwarfed by the likes of France. Quality and not quantity of troops should be on the focus here. People that only know of Ottomans through EU4 would have a very wrong image of their military now, and this would be a good opportunity to fix that all the while fixing their OPness problem as well.
5) Since my BA thesis was on the Black Sea slave trade, I suppose a mechanic revolving around that and that is in connection with the Mamluks, Genoese/Venetians, and Ottoman conquest of Constantinople can be put in game too. It is hard to say what exactly, but Mamluks that have a hostile relationship with the owners of Black Sea slave ports and/or Constantinople should receive severe maluses to its slave-based society that relied on Italian merchants ferrying them from said ports. They may be required to make a land route to Caucasus themselves to get rid of these maluses. Ottoman players should have the ability to stop this trade if they wish to, and piss off the Mamluks whilst weakening them in the process. Of course, this should also put a huge dent in Ottoman economy too.
