I dont think Marvel Comics is necessarily a bad choice for a crossover, even if its mainstream relevance has fallen a notch or two from its Avengers heyday
its an endearing bit of worldbuilding that makes Theros feel like a rich, breathing world, instead of just some artwork in a trading card game
Outpost Siege has another option if you dont need a card advantage
An ideal first deck would contain a majority of the mechanics that Lorcana has to offer while containing many different strategies to explore

3 Braska's Final Aeon Cecil, Dark Knight Cecil, Redeemed Paladin Chaos, the Endless Choco, Seeker of Paradise Cid, Timeless Artificer Clive, Ifrit's Dominant Cloud of Darkness Cloud, Midgar Mercenary Cloud, Planet's Champion Diamond Weapon Dion, Bahamut's Dominant Edgar, King of Figaro Emet-Selch, Unsundered Esper Terra Exdeath, Void Warlock Fang, Fearless l'Cie Firion, Wild Rose Warrior Freya Crescent G'raha Tia Gaius van Baelsar Galian Beast Garland, Knight of Cornelia Garnet, Princess of Alexandria Gilgamesh, Master-at-Arms Giott, King of the Dwarves Gladiolus Amicitia Gogo, Master of Mimicry Golbez, Crystal Collector Hades, Sorcerer of Eld Hope Estheim Hydaelyn, the Mothercrystal Ifrit, Warden of Inferno Ignis Scientia Jecht, Reluctant Guardian Jenova, Ancient Calamity Jill, Shiva's Dominant Joshua, Phoenix's Dominant Judge Magister Gabranth Kain, Traitorous Dragoon Kefka, Court Mage Kefka, Ruler of Ruin Kuja, Genome Sorcerer Lightning, Army of One Lightning, Security Sergeant Locke Cole Matoya, Archon Elder Minwu, White Mage Neo Exdeath, Dimension's End Noctis, Prince of Lucis Omega, Heartless Evolution Phoenix, Warden of Fire Prompto Argentum Queen Brahne Quina, Qu Gourmet Quistis Trepe Ragnarok, Divine Deliverance Raubahn, Bull of Ala Mhigo Reno and Rude Rinoa Heartilly

The Scarab God- One of the three forgotten gods re-made by Nicol Bolas