I'm gonna try to post a piece of Deltarune fanmade content every day until about June 5th in the lead up toe Chapters 3 and 4 being released. A ton of fans have made some insanely cool content or concepts in the last three years since Chapter 2 was released.
For starters, remember when this guy was who everyone thought was going to be either Chapter 3's main villain (which would've made Spamton's mention of "Mike" line up with Jevil's warning about Queen in Chapter 1) or Secret Boss (theorized to be because of his association with Spamton himself)?
While this track and the fangame it was made for are insanely good, I'm not sure how well the song would fit in Deltarune. It's got some distinct leitmotifs that are both original as well as taken from the game itself like the "Freedom" leitmotif, but the song's sound font and audio quality feel a bit at odds with some of the other tracks in the game, and it's melody is wild and all over the place. Still, I could see a track similar to it potentially being in the game give that Deltarune so far has had an even bigger variety of drastically different-sounding tracks than Undertale.
On the other hand, I think the design for Mike here would be a bit of a letdown in the actual game. The eye atennas are cool, but the rest of his design here basically boils down to just being an animated version of the mysterious face sprite that appears in the mansion attached to a TV screen like what ended up seemingly being part of Tenna's design, and two giant floating hands ala
many Nintendo bosses. It's not bad, but it feels a bit odd and unpolished compared to the known official Secret Bosses and so many other fan designs for Secret Bosses that have been made since.
Whatever the case, I'm looking forward to the next Chapters immensely! I think I might actually like Deltarune even more than Undertale itself so far.