Karn, Scion of Urza
Legendary Planeswalker — Karn
+1 Reveal the top two cards of your library. An opponent chooses one of them. Put that card into your hand and exile the other with a silver counter on it.
-1 Put a card you own with a silver counter on it from exile into your hand.
-2 Create a 0/0 colorless Construct artifact creature token with "This creature gets +1/+1 for each artifact you control."
This seems quite strong to me; even if the +1 is only drawing you extra lands, you can get back any especially powerful spells you exile (note that Karn can grab cards exiled by other Karns), Karn can protect itself with the -2 ability, and the creature generated is of relevant size in any deck with sufficiently high artifact count. The main problems is that Karn is not himself an artifact and so cannot be powered out by Workshop.