Dies to Removal

I like Seizan as a way to safely set up to go off in mono black decks. (The combo oriented ones of course, like Shirei or Drana.)

He gives you the two things you need in that deck; time and resources.

Steel Hellkite is a pretty good card universally. I play it in pretty much every deck because of how good it is against token strategies. That said, I wouldn't mind playing another way to tutor for it in mono red.

Weathered Wayfarer is a big one in white.

You could probably get away with adding X Sun's Zenith to every color too, since basically all of them are playable to some degree. Red is the only borderline unplayable one.