More Ruins!
(pencil, scanned. Again, the bricks look pinker than the physical version but whatever)
In the original UT, Toriel gets food and necesities from Home (the city), according to that one Froggit just before the balcony room where you find the toy knife. They say “I saw [Toriel] come out carrying some groceries (...)” so I always assumed the Ruins are simply a separate area to Home, which is still inhabited by monsters.
However, I guessed with the door to Snowdin locked it would make more sense for Home to be abandoned. Toriel and some minor monsters are the only ones wandering the Ruins, aside from Napstablook who sneaks under the doors. So for all of them to survive they’d have to harvest their food somehow.
Which is why I’ve decided to deck most of the rooms in the Ruins with 1) more water cannals and 2) raised plantations. Toriel might not be good at gardening, but she can keep crops alive after having to live through the harsh conditions of the war against the humans.
Also, as a direct result of more water cannals and the layout being slightly different due to the plantations, most of the puzzles would deal with changing water levels and water-related traps like moving platforms and such.