I hath summoned myself to explain in detail.
Almost every song in Undertale is connected by the motifs. Either one of the songs with the motif connects with another motif (ie Undertale connects several of them) in ways that make sense. There are a few songs/motifs that don't tie in with this. The Uwa motif is only connected to the uwa songs, and doesn't connect to the rest of the track. The few songs that have no leitmotifs are normally the smaller songs; Unnecessary Tension, Dangery Mystery, Thundersnail, Chill, and a few others. The only two "full" songs without leitmotifs are Wrong Enemy; which makes a bit of sense, So Sorry is such an obscure thing; and Megalovania. Megalovania, despite commonly being associated with Snas, does not share any connections with his theme. There is one portion of Bring It In, Guys! that kinda sounds like a portion of Megalovania, but it's a stretch and I personally don't hear it myself. Song That Might Play When You Fight Sans has the motifs of sans. and Bonetrousle. Again, it's never used in game, but Toby left it in the OST so that people would think that there's a neutral Sans fight. So, it's just Sans once more trolling us across Space and Time. Megalovania is different, because it's not Sans' fight theme. It's your theme. The definition of megalomania is "obsession with the exercise of power," or "the delusional belief that one is important, powerful, or famous." You got to this point because you pushed the game to it's limits. Sans isn't fighting you, the game is. You're fighting the game itself. It threw an emotional barrier at you, Toriel and Papyrus. It threw a skill barrier at you, Undyne. Now, you fight someone who breaks the rules of the game to survive. This isn't about the funny bone man, it's about YOU.
With the inclusion of the 10th anniversary and Song That Plays When Someone Versus Sans, it does have Megalovania's motifs in it. However, it feels much more fun than Megalovania. I say this is because it's still the player's theme, but this fight isn't to decide the fate of the game; it's for charity. You're here for fun.
