Legality depends on the tournament format -- Coat of Arms can be legal in some formats and banned or restricted in others
Some collect tournament promo cards with specific stamps on them, and others also collect trophy cards, which are given to tournament winners

Beyond those suggestions, it really depends on what you think the deck is lacking the most: Red Wraths: Blasphemous Act, Rolling Earthquake Flash: Vedalken Orrery, Winding Canyons Sac Outlets: Ashnod's Altar, Phyrexian Altar, High Market Removal: Swords to Plowshares, Path to Exile, Grasp of Fate, Council's Judgment Card Draw: Wheel of Fortune, Mind's Eye, Chandra, Flamecaller, Skullclamp Ramp: Solemn Simulacrum, Sol Ring, Chromatic Lantern, Coalition Relic, Chrome Mox, Mox Opal, Mox Diamond, Mana Crypt, Mana Vault Protection: Boros Charm, Avacyn, Archangel of Hope, Glory, Flagstones of Trokair Tutors: Enlightened Tutor, Weathered Wayfarer, Tithe, Idyllic Tutor, Land Tax, Stoneforge Mystic, Godo, Bandit Warlord, Expedition Map Stax: Winter Orb, Static Orb, Cataclysm, Armageddon, Ravages of War Anti-Control: Stranglehold, Grand Abolisher, Price of Glory, Homeward Path Lands: Clifftop Retreat, Rugged Prairie, Plateau, Sacred Foundry, Arid Mesa, Strip Mine, Wasteland, Mistveil Plains (lets you re-use cards with Sunforger) I don't often do this, but here's my take on a blinged out Archangel Avacyn, where money isn't an obstacle: We've got the White Value Package for card advantage / ramp / library manipulation, a Sunforger Package that can be re-used with Mistveil Plains, Mass Land Destruction once we're ahead on the board, and numerous combo combinations involving Adarkar Valkyrie, Kiki-Jiki, Mirror Breaker, and Ashnod's Altar

Inspecting the edges with the naked eye is done by seasoned Pokemon collectors because they know what to look for
To that end, Set 4 featured a separate side-game you could purchase, 'Illumineer's Quest: Deep Trouble,' which put a neat cooperative spin on the standard Lorcana gameplay as you and a chum worked together to take down an automated Ursula deck
The illustration depicts the dark-type bird soaring through moonlit skies with purple-black flames trailing its wings a dramatic reinterpretation of Moltres that captures Galarian variants sinister identity