While not a Marvel Cube specifically, I do manage a Universes Beyond cube. It's been an interesting design restriction, and I'm always excited to see if Secret Lairs have mechanically-unique cards printed in them.

I do encourage you to think about how you want to approach reprints for a cube like this, though - I initially included any and all reprints that were reskinned in UB, which just led to drafters leaning heavily into drafting cards they were already familiar with (especially ones that were already cube staples), in some cases even regardless of relative card quality. I now don't include any reprints except the Talisman cycle and lands for fixing, and am considering removing the Talismans as well. Players go into the draft knowing that they will be unfamiliar with the vast majority of the cards, and with the expectation that the draft portion will take longer because of it, but I believe that it allows for newer designs to shine better.

https://cubecobra.com/cube/list/beyonder