devtrackers.gg
Or they just added a few random high-cost cards for Momir so that you can't play the same creature indefinitely at high costs where there's only one or two creatures available in that mana slot.
^ This. These cards are just to add some spice to Momir at high CMCs; no connection to the new cards that will be added for Historic.
Since you guys don't seem to have a problem with adding older cards for a specific event, is there any chance we'll see a form of Cube show up? Even if it was a Pauper cube with Kaladesh+ commons it would be really fun.
Some cards are easy to add; they “just work” and we don’t need to do anything special to support them. Other cards take real work (sometimes a lot) to get working properly. Since Momir is a random and scattershot mode, it was fine to just add in whatever works automatically. Cubes, on the other hand, are artful and carefully constructed. Just using “whatever cards worked automatically” would make for a pretty poor one.
So we can’t really pull the same trick for a cube, but figuring out how to support cubes properly is something we’re definitely interested in. It’ll just take a bit more time and effort to pull that off than this did.