Some ideas I have about what Detective Conan's final might look like:
-Conan discovers the truth behind the events of the Karasuma mansion (chapter 300): Renya Karasuma faked his death and arranged for all his rivals to reunite during the auction 40 years ago. Then, everything was arranged in order to murder them, allowing him to take complete takeover of the criminal underworld, while taking back his precious belongings (his fortune), without any investigation being in charge considering the unusual circumstances of the event.
-Conan and Haibara confronts an 140 years old Renya Karasuma about his experiments, trying to achieve eternal life. The experiments have succeeded to maintain him in life this old, but now he seeks a more permanent solution: eternal youth. The boss is convinced that eternal youth can be achieved since one case was a complete success (Vermouth), allowing him to hope for himself, but Vermouth sees eternal life as a curse and she hates Karasuma for what he did to her. Haibara learns that Vermouth was the one who assassinated her parents in revenge for what they did to her and in order to prevent them to complete their researches.
-Vermouth intends to kill the boss but Karasumu kills her before she even has the chance. In her dying words, Vermouth says that she is happy her life finally comes to an end since it also marks the end of her existence as a monster during which she has forgotten who she was truly. She says that she is happy the son of Yukiko is at her side by the end and that he is "a true silver bullet". Conan finally understands what she means by that.
-To confirm that the APTX-4869 can truly rejuvenate someone, Karasuma uses it on Ran, held hostage. She rejuvenates, becoming a 6 years old girl again. Confident, the boss is about to take it while Conan tries to prevent him. Then, the boss begins to slowly dying and wonders why. Conan reveals that Ran, Haibara and he share the same blood type and it is so rare that the chances the boss would rejuvenate were pretty low. In the last moment of the boss, Conan says to him that, while he can't forgive Karasuma for all the suffering he has brought on too many people, he would have try to save him, since a detective who lets a culprit kill himself isn't different from a murderer. The "silver bullet" (APTX-4869) has finally killed the monster.
-Haibara consideres stopping her researches on the cure since it could lead to the achievement of what the organization was seeking. In doing so, she finally ends the cycle of suffering her parents have begun many years ago. Too many people have already suffered enough because of it, and Shinichi no longer needs the cure since he's finally reunited with Ran after all this time, as 6 years old children in school. Haibara can now finally live the life of any normal young girl with all her friends at her side.