The fact that there is a bunch of mechanics from various sets throughout magic history. Callbacks to old cards, even Alpha stuff. Twists on existing cards. Time Spiral block was about the past/present/future of Dominaria all meshing together. Time Spiral was the past, Planar Chaos present (with color pie swaps, i.e. damnation was just a black Wrath of God), Future sight the future (many of the future card frame cards ended up being printed in future sets).
Some examples:
That red goblin revealed recently, Pashalik Mons, is a deep-cut reference to Alpha card 'Mons' Goblin Raiders'. He is directly referenced in the flavor text.
That Vesperlark card is a 'baby Reveillark'.
Urza/Yawgmoth/Serra are 3 of the most requested characters to get a card, all 3 with crazy amounts of lore and story going back to the earliest sets.
Bazaar Trademage is 'Bazaar of Baghdad's effect on a Serendib Efreet, with flavor text referencing cards from Arabian Nights.
Deep Forest Hermit is Deranged Hermit with Vanishing.
Ravenous Giant is a red Juzam Djinn
etc..