Curiosity and related effects are incredible with creatures that tap to deal damage
Mishra has a very specific synergy that makes it work really well: You need to have Mishra, Artificer Prodigy and Possibility Storm on the field
Heres the stuff I think is interesting and not just Multi-color manabase card. Before I do that, I just want to point out that every one of these decks will have Arcane Signet and anytime someone builds a new deck, theyll need a Signet from a finite current supply, so anything that makes people build a new deck between now and a Signet reprint is significant with respect to that card
In comparison, cards like Sphinx's Revelation or Expansion // Explosion could provide similar benefits more efficiently and are commonly seen in control decks, making them potentially better options for inclusion in a competitive deck
The only time the ulti is bad is when you are playing a beat down deck with basically no real top end
If you see these in a booster draft, make note--it wont win you games, but it will help subsidize your entry coast a decent amount