Obvious 4 of in every deck. Sightseer is to Sycamore as Guzma is to Lysandre. The card is is excellent for all decks that want to selectively shove things in the discard. It will also be nice to save key items.
probably not useful if you have over 5 or 6 cards, because you end up discarding and not drawing. though you can discard below 5, and sometimes sycamore is more useful(in expanded) and even sophocles is better. only useful if you can discard enough cards to justifying draw up to 5 cards. i can see this in expanded, dark patch, aqua patch, malamar>?
i can see this being useful a nm deck, where you have too much nm pokemon and you dont want to discard other essential cards in your hand. useful for alolan exeggutor as well.
if you have have 7+ cards in your hand sightseer essentially becomes a dead weight card that will only discard from hand.
Magcargo can be used in any deck, not just fire, although you might have to attack with it?
2-2 magcargos-gx and baby magcargo is good.