Since you like theories I’ll give you one what are your thoughts on Gaster possibly a Darkner or Darkner/Skeleton hybrid?
dang it i was supposed to answer this the other day but i was busy! >_< sorry!
well! i was totally on board with the whole ‘sans might be a Darkener’ thing, so i think it’d be awesome if Gaster was too! i did a very speedy run through deltarune because i had work lol so i didn’t manage to see all the references to him T0T
my own personal theories about gaster are so mixed, though, since there is precious little known about him! at first, i didn’t see him as a skeleton, but then people pointed out the WD in his name possibly referring to the text ‘wingdings’ for his speech, and since sans and papyrus ‘speak’ in their namesakes, i kind of liked the idea! I dont know about his relation to sans and papyrus though…but many fan theories that connect the three are really cool [i especially like leafaske’s gaster bros comics!]
for ages i thought that maybe he injected Determination into himself, which is why one half of his face is all droopy - because monsters with Determination in them basically melt - and that the famous Entry 17 of the true lab was actually about himself……….but i think i was super wrong lol, it may have been about Flowey……
and then there’s the whole ‘he was ripped across time and space’ thing - i always figured that Determination - the ability to reset - is basically messing with time and space……..sooooo………..
then again, a lot of people theorise that he fell into the CORE
heck dude i love theorising about gaster!! sorry i went off on a bit of a ramble there! but your question got me kinda hyped :D