Workshop decks have been a constant staple in Vintage, and this is one of Magics greatest lands
Agathas Soul Cauldron Chief: For just two mana, you get an artifact that color-fixes the mana for activated abilities on your creatures and a piece of graveyard hate that can put +1/+1 counters on creatures when you exile creature cards
A combo can be any set of cards which effectively ends the game in your favor, be that a prison like Teferi + Knowledge Pool, infinite damage/mill, or even a very fast combat clock like Skithiryx, the Blight Dragon + Lightning Greaves
Madison Li's effects use mana to cast, so as long as you have the energy counters to pay, you can use any of them depending on what you need for the gamestate
Due to the way in which white removal is split I am happy with a couple of removal spells that are narrower in my cube provided they have this level of power