Board Thread:Wiki Discussion/@comment-31960778-20170721040405/@comment-32182236-20171224124701

Well, let's just say Undertale would be even more self-aware than it already is. But, it wouldn't be so balanced.

The Ruins would work the same way, but when you get to Snowdin, things would change-Sans and Papyrus would have moved to Hotland with Gaster. (Their origin is unknown to anybody in canon because it no longer exists. Hint, hint..)

After you fight Undyne, you'd meet Gaster himself in the lab. Considering his nature, he likely wouldn't approve of Alphys repurposing Mettaton-He'd remain as a human eradication robot, meaning that Mettaton would always be Mettaton NEO. And let's just say Gaster would have been doing some research on spacetime, like before he died.

"Our reports showed a massive anomaly in the timespace continuum"-Sans

OUR reports. Considering the strange text in his lab, and the badge, he likely worked with Gaster. That also explains the "don't forget". "Our reports" most certainly included Gaster. Therefore, you'd probably end up fighting against Gaster himself, as he does things similar to that of Sans, using loopholes within game mechanics against you.