The title says it all. No regulations on the PoD.
Here is an easy POD. On the 12 July 1867, Prince Abdul Hamid (son of Sultan Abdülmecid I) accoumpanied his uncle, Sultan Abdulaziz on his visit to London.
While in London, the 27 year old Prince begins suffering from abdominal pain, diarrhea and a fever. By 20th, there were signs that he had lost a dramatic amount of weight. With Punch, placing a cartoon commenting on the prince's health.
A doctor was called for, but to no use, Queen Victoria, mentions that these are the same signs of her husband, Albert, who had died 6 years before and sure enought on 6th August, Prince Abdul Hamid, also fell victim to death's cold boney had, his body was sent back to Istanbul.
With Prince Abdul Hamid unable to take the thone in 1876, the empire had only a handful of Princes left to take the title, however most of them were reformist wanting implementation of democratic reforms.
So your first Democratic-Sultan is between Murad V or Mehmed V, both elligable.
With these two in power, the Ottoman constitution will be written in 1876, like OTL, but without Abdülhamid ultimately suspending the constitution and parliament in 1878 in favor of a return to absolute monarchy with himself in power, this time, Namık Kemal is placed as Prime Minister with the Young Ottomans being a popular early Socialist party fighting for freedom and fatherland within the Empire.
This is the earliest I could do with out going into Post-1900
