An X-themed deck lends itself to a lot of neat card choices, since restrictions can be the basis of innovation
MagicFest promos are named Festival, MagicFest, Commander, Play Promo or something completely different in the legal text
This is likely because full art cards have fully rendered backgrounds and proper coloring, which tends to be favored by casual collectors more than the sparkly rainbow filter of the Rainbow Rare cards
And the most terrifying of all the Eldrazi is Emrakul, the Aeons Torn
1 Traditional foil rare or mythic rare that's an extended-art, borderless, or retro frame card, or one of the following: A double rainbow serialized Eldrazi (250 of each) A foil-etched rare (9.3%) or mythic rare (3.9%) A traditional foil Special Guests mythic rare (10 cards total4.4%) A textured foil Special Guests Elemental (1.9%) or DFC planeswalker (1.9%) The remainder of this slot has the same content as the non-foil slot above with the percentages reduced appropriately to account for the inclusion of serialized, foil-etched, textured foil, and Special Guests cards
Another option is Aetherworks Marvel: Creatures (15) Planeswalkers (2) Lands (20) Spells (23) This deck also checks many of the Brainstorm boxes it naturally plays shuffle effects (Fabled Passage and Attune with Aether) and it specifically has combo pieces that it wants to get rid of and put on top of its library