Specifically, the end of Chapter Black and Three Kings. As much as I like the idea of Yusuke fighting a former Spirit Detective being the finale of the series, ending the series without exploring what the Demon World was like would have been rather unsatisfying. The next arc taking place in the Demon World was a logical progression from Chapter Black, which centered around the portal to the Demon World being opened. The main issues were the bullshit retcon that Yusuke had demon ancestry and the half-assed tournament (how the fuck do you skip Yusuke vs. Yomi, which was supposed to be the final fight of the manga?).
Here's my idea. In the Chapter Black arc, instead of Sensui killing Yusuke, he kills Koenma; right when Sensui is about to kill Yusuke, Koenma jumps to take the blow and dies. The series can unfold in two ways.
1) Enraged over Koenma's death, Yusuke powers up to Class-S and kills Sensui. Itsuki takes Sensui's corpse and leaves. Koenma later gets brought back by his father Enma (seeing as Koenma's pacifier could revive the Game Master kid, Enma could have his own tool that brings the deceased back to life), the portal is sealed and the series ends.
It's a rehash of the Dark Tournament's ending, but given Togashi's ailing health, it would have been better if he just ended the series here. If he really had to continue it, though, then it should have gone like this:
2) Enraged over Koenma's death, Yusuke, Kuwabara, Hiei and Kurama all get rage boosts that power them up, but none of them reach Class-S. They fight Sensui in the Demon World and lose. Sensing Yusuke's desperation to defeat Sensui, Raizen partially turns Yusuke into a demon against his will. The partial demon Yusuke snaps and accidentally kills Sensui, whose corpse is taken by Itsuki. Yusuke feels guilt for having killed Sensui, who was human. The portal is closed by Enma's special force, leaving Yusuke, Kuwabara, Hiei and Kurama stuck in the Demon World. Raizen, the strongest of all demons, is the only one capable of opening the portal. He'll only open it for Yusuke and his friends to go back to Human World if any of them can defeat him. Yusuke trains with Raizen's men to surpass him. Yomi and Mukuro are the strongest demons after Raizen. Kurama trains with Yomi and Hiei trains with Mukuro. Kuwabara trains with Jin, Chu, Rinku, Shishiwakamaru and Suzuki at Yomi's territory.
As Yusuke trains to surpass Raizen, he gradually loses more and more of his humanity. It's suspected by Yomi and Mukuro that Raizen's true plan is for Yusuke to fully become a demon and have him destroy the Human World himself. A year passes and Yusuke is sent to kill Kuwabara, Kurama and Hiei to fully complete his transformation into a demon. Yusuke battles his friends and defeats all of them, but Kuwabara brings him back to his senses, which fully returns him to his human form. Yusuke still has the same amount of power he had as a near-complete demon (the Demon Energy is converted to Spirit Energy) and he goes to fight Raizen. Yusuke powers up further by awakening Sacred Energy and defeats Raizen, who reveals that his true goal was to be killed by a human as penance for all the humans he killed in the past. This parallels how Sensui's goal was to be killed by a demon as penance for all the demons he killed in the past. Raizen anticipated that Yusuke would regain his humanity and the reason he didn't turn Kuwabara into a demon was so he could help Yusuke. Yusuke, however, doesn't kill Raizen, declaring that both humans and demons deserve to live. Raizen opens the portal and Yusuke, Kuwabara, Hiei and Kurama return to the Human World. Koenma is brought back to life by his father. Series ends.