Oktagon left out one major fiend in the D&D universe- Demogorgon. Probably one of the most iconic antagonists in all of AD&D. If you are going to include Vecna, Orcus and Asmodeus, you should have included the greatest demon of all time.
They can only make cards from the paper MtG set, and they didn’t print the Demogorgon in the set.
And while this didn’t make the set in MTGPQ, it does represent the only option they would have even had.
That’s the one I’m most familiar with too. I probably still have that Monster Manual buried somewhere in the barn.