Ah The End, the final boss that's stirred up the most mixed emotions I've seen from fans in quite a while. To me the fight is unique, but not overly grand. But the final fight is more of an epilogue than a big fight like Dark Gaia or Solaris, it's the culmination of the new era of Sonic and what's to come, and it's strangely kind of left to everyone's interpretation.
As far as mashups go End of the World struck me as a big possibility to go with this from both a musical standpoint and a "Impending doom" kind of vibe.
A Mashup of "I'm with you" from Sonic Frontiers, and "End of the World" from Sonic 06.
Infinite background by LegendQueen01 on DeviantArt
Super Sonic photo by Legitamategamerz on DeviantArt
Mephiles photo by Nibroc-Rock on DeviantArt