From the opening moral argument to each and every tense mental standoff, Death Note brilliantly crafts a gripping and dramatic plotline. While some argue that the series loses traction in later segments, I find no serious faults. But in the face of questions on what could have been done differently, I'll play along and purpose my concepts for a revised second half.
Currently we have Near as Light's chief antagonist, with Mello acting as a thorn in his side. Simply switching those roles opens up a whole new outlook on the second half. In chapter 67 of the manga, while Mello is in possession of a notebook, Near stipulates to Light that maybe "like Kira, Mello is trying to take over the world." Interesting.
If Near vs. Kira is too close to the previous match-up, then Kira vs. Kira should be the fresh plot device we need. Near's stipulation -- only brought up to test Light -- is currently wrong, but we don't need it to be right. Let Mello's intentions be a big, blurry grey line between surpassing L and Near and seeking out too much power of his own.
We've already seen Mello play dirty and abduct Light's sister in his quest to attain a notebook. With a lower moral bar and more power to abuse than L, the second half of Death Note can now take a darker, more emotional tone. In the end, Mello can fall the same way he does now, as Light is forced into sacrificing his own father to get back that notebook.
Now, Mello's time bottom feeding off of Near after his defeat will have more meaning, with the humiliating loss of power more evident. His drastic action of kidnapping Takada becomes even more pitiful. However, note that even allowing Mello to use the notebook against Light possibly contradicts the manga's final chapter.
If Near really beat Light by using the notebook as postulated by Matsuda, a nice theme emerges that Light could only really be defeated by someone using his own power against him. However, since the anime and manga differ in that scene even existing, we can let the anime adaptation be different, and thus allow Mello to use the Death Note against Light. The potential battle between two would-be gods might be too good to pass up.