Sephiroth wants to transcend time and space. He wants neither the end of being diluted into the lifestream,
nor the end of being swallowed up by a supernova. It's not enough to become a god with the Meteor plan, for then even after becoming one with the planet Sephiroth could still be destroyed by catastrophic astronomical events. He wants to become a guaranteed
eternal god.
I interpret the ending of FFVIIR as Sephiroth using Cloud to defeat the Whispers, which defeats one obstacle towards eternal godhood. Despite Cloud just having defeated "destiny", unwittingly doing so on Sephiroth's behalf, Sephiroth STILL says at the edge of creation "
Lend me your strength. Let us defy destiny, together." to Cloud.
This is not a request for Cloud to just be by Sephiroth's side because YAOI BAIT. There is still some obstacle left towards Sephiroth fully transcending time/entropy. Some entity or thing that Cloud needs to defeat/remove, same as Sephiroth needed Cloud in order to defeat the Whispers.
Same as Square went bonkers with the inclusion of the Whispers in FFVIIR, they may go bonkers with who/what the next thing is that Cloud & team must defeat so that they "defy destiny", just as Sephiroth planned. Will Cloud battle the Time Guardian, whom I speculate is the originator behind the Whispers? Will Genesis be acting as an agent of the planet's will, either waking up early or arriving from the future to try and stop Sephiroth's plan? That'd certainly tie up the loose end of Dirge's secret ending if FFVIIR Part 2 or Part 3 ends with us defeating Genesis.
Sephiroth wants from Cloud to be a puppet for his plans, same as always. It's also entirely possible that this Sephiroth has gained such a deep respect for Cloud that he wants them to share eternity together, but that idea is much more uninteresting compared to the actual scheming that's involved here.