I do not think Sans will be the schizoboss of Chapter 5. This is the same thing as everybody thinking the rest of Deltarune will be Card-based after Chapter 1, or making every schizoboss a washed-up/discarded item after Chapter 2. Sans would be Gerson, again. I do think there will be a blue soul fight, but it will be on Chapter 6 (all Undertale callback bosses are on even chapters), and will be closer connected to Papyrus (since all the Undertale callback bosses are "what-ifs," like a completed NEO or Gerson in his heyday, and we've already seen a complete Sans fight).