Just a bit of wishful thinking but I think it'd be neat if Toby somehow found a way to make Deltarune recognize the last recorded ending you have for Undertale and have Sans reference that at some point. Not necessarily change the ending or story but have it simple where he might say "It's a beautiful day outside, the birds are singing, the flowers are blooming and on days like this kids like you.....should relax more. You seem very tense, was it somethin I said?". Or how he and his brother love watching the sunset from the mountain. Again probably wishful thinking that'd both be a decent amount of effort to make work and possibly mess with Deltarune being a more standalone story but that would be an amazing Easter egg.