These are Reserved List or high-demand staples
Opalescence (Ext, S) The weaker half of the Replenish-Opalescence combo, nevertheless this card terrorized the Extended and Standard scene for long enough
There is the the combo direction as well which links this archetype up to things like the Birthing Pod creature combo decks
Coram, the Undertaker Coram, the Undertaker can take advantage of your mill abilities
Granted, those decks are almost all focused on milling out the opponent but the original Hedron Crab is also able to target yourself, which makes it one of the most explosive and mana-efficient self-mill engines ever printed
The first step is deciding our overall vision for the deck, so we can make sure our cuts and additions are coherent