There aren't many Expansion colors left...

Each one of these colors has lighter and darker, more saturated and less saturated shades. Also, I bet black will be used at some point since it's actually quite radical from the rest of the colors. All this can be done while staying away from the specific shades of blue and green of GPs and SPs.
https://i.imgur.com/GDxm2ek.jpg

I don't think they'll be running out of colors, no ?