Warning: Spoilers for Lupin Season 1.
Lupin and Sherlock Holmes have a lot in common.
Lupin is based on Maurice Leblanc’s collection of stories that star Arsène Lupin as a ‘gentleman thief.’ Leblanc wrote dozens of stories based on the character Arsène Lupin between 1905 and 1936. Leblanc, who was born in Normandy, in 1864 was a French contemporary of Sir Arthur Conan Doyle, who was born five years earlier. Leblanc was intrigued by Conan Doyle's famous English detective and decided to include the character in his own works.
Leblanc first included Sherlock Holmes in the short story "Sherlock Holmes Arrives Too Late," which was published in 1906. After Sir Arthur Conan Doyle objected to Leblanc’s usage of the character, Leblanc changed the name slightly. Herlock Sholmes appeared in two more stories and the name was changed once again to Holmlock Shears in 1910. Since Leblanc introduced the character into Lupin’s universe, it’s plausible that Lupin: Part 2 could include the character – though, at least according to the Conan Doyle estate, whether Sherlock Holmes has entered public domain is still up for debate.
If Lupin did decide to introduce Sherlock Holmes, it’s more likely that the character would be an homage rather than an exact replica – and the character almost definitely wouldn't be played by Robert Downey Jr., Benedict Cumberbatch, or Henry Cavill. More importantly, there are more pressing matters at hand for Lupin’s characters, namely, rescuing Assane and Claire’s son, Raoul. There aren’t any plot details available for Lupin: Part 2, but it would make sense for the series to wrap up loose ends from Part 1 before tackling other stories. However, as Part 1 proved with the character Fabienne Beriot, it’s still possible to include a compelling minor character without overshadowing the larger story.
A crossover of the two series could work, but Lupin has proven that it doesn’t need to use Sherlock’s popularity to attract viewers and Assane can certainly stand on his own without Sherlock’s (or Herlock's or Holmlock's) expertise, though Assane would probably accept any help finding his son. It’s unlikely that the crossover will happen because it could come off as “gimmicky” and detract from Lupin’s already fantastic story and characters – but as unlikely as it may seem, it’s still possible. After all, Arsène Lupin and Sherlock Holmes crossovers have happened before.