What do you think of the idea that Megalovania is Chara's battle theme rather than Sans? I
there’s something that makes me question it in a big way, and that something is the existence of the unused track, song that might play when you fight sans. it contains elements of sans’ and papyrus’ themes, among other things. although it may have been intended for a neutral route battle, the fact is that sans was always intended to have his own battle theme. not to mention the fact that every other boss has their own unique battle theme connected to them.
of course, megalovania has been used before, including an earthbound hack. so the name itself is probably not enough to give it away in any case. but if we focus on the name alone, we find something interesting. the song name could refer to the word megalomaniac - a person obsessed with the exercise of power, especially in the domination of others. that description doesn’t fit sans, which is fine because the song wasn’t originally made for sans.
so who in this game is a megalomaniac? the most likely candidate is chara.
as the player runs through genocide and chara grows stronger, chara makes sure the player eradicated everyone. when the player tries to leave waterfall to fight undyne, if the player has not eliminated the quota of monsters, chara will notify the player of this and suggest to not leave yet.
at the end of genocide, chara implies the player was never in control, that chara is the one in power. chara very well fits the description of megalomania. even if they aren’t happy with multiple genocide runs (”i cannot understand these feelings any more”), they take the effort to keep track of how many enemies are left to kill regardless.
but chara isn’t the only megalomaniac. the player is one, too. in a genocide run, the player is destroying everyone because they can. after all, the player is trying to prove they can beat genocide, and in many cases specifically sans. they exercise this power to dominate and beat the game. what else is a player trying to prove when they go after sans again and again?
however, this is only speculation driven by the meaning of the name. and the song existed long before undertale was created. it is possible that toby just decided to use this song as sans’ battle theme because of how amazing it is.
personally, i like to think of once upon a time as chara’s theme, considering it plays while showing their journey in the intro. since chara is connected to everything in undertale, and elements of once upon a time can be found in many themes, it makes some sense. the idea also makes hopes and dreams seem a little more emotional – it wouldn’t just be asriel’s theme, but chara’s, too.