Rain started off as a Troll. From the MK wiki
Rain was originally meant to appear in Mortal Kombat 3, but didn't actually appear until Ultimate Mortal Kombat 3. Rain was originally a red herring inserted by the game developers; a character shown in the game's attract mode who did not actually exist within the game itself. A recent interview reveals that the insert was made by Ed Boon himself to intentionally trick players. Many MK fans searched for him in vain, with the eventual realization of his status as a joke character (named after a Prince song, "Purple Rain") dawning. The joke expands further when it is revealed that Rain is a "prince" himself.
So just because the Chrome Kombat Kard is a joke that doesn't exactly mean it's not gonna happen.
The difference is that Khrome is a fan-created character, which is the point that Tarkatan and I are trying to make.
Whether you like it or not, PCS Collectibles don't work for NRS, and the stuff that they create (which they don't have a license for) can be considered "fan made."
They didn't have a license to make a Khrome statue.
They had licenses to make Retile, Sub-Zero, Scorpion, Smoke and other statues.
One day some shiny metal accidentally fell on a Smoke statue, and
a guy that works for PCS Collectibles decided to call him Khrome.
Ermac, Skarlet and Rain are all characters that were either errors, glitches and / or trolling perpetrated by NRS (Midway at the time).
Never in the history of MK, as far as I know, has Midway / NRS ever gone through with a fan creation.
There could possibly be legality issues with NRS putting in a fan-created character, so they likely don't bother with it at all.
Again, the fact that it was something that wasn't created by NRS and rather "fan made," to begin with, means that the chances are very unlikely for that to happen.
I'm pretty sure that they are covering their ass and not taking their chances, especially after the legal battle that Midway had to go through before WB acquired the MK assets and re-branded the development studio as NetherRealm Studios.