You could consider
Card draw
Id personally cut down to maybe 36 lands and add a few mana rocks in. Definitely at least a Sol Ring
EDHRec is a good reference for what other cards people are running and synergise well
edit - for the specific theme of abusing gandalf commander damage, ghostly flicker and other similar cards so you can reuse the “choose one not chosen”
The phrase "that hasn't been chosen" refers only to that specific Gandalf the Grey. If Gandalf the Grey leaves the battlefield (possibly because of the fourth mode of its ability) and then returns to the battlefield later, it will be a new object with no memory of the modes chosen when it was last on the battlefield.
edit as another comment has mentioned, Gandalfs 3 damage pings don’t count for commander damage
https://draftsim.com/mtg-commander-damage/
Commander damage is a pretty simple rule. When your commander hits a player for combat damage (i.e., only during combat when the commander is attacking and is unblocked), note how much damage that player has taken from your commander. If a player has been dealt 21 or morecombat damage over the course of the game, they lose.