Currently testing: Maze of Ith #3 as 61st card, Mirage Mirror in place of relic, and Nephalia Academy in place of Ancient Tomb
(update on testing: I'm actually quite happy with maze of ith and having 61 cards in the deck, there are a lot of one-ofs that you really don't want to draw first off, and maze of ith is a card I almost always prefer to draw rather than search for. However, I'm only confident about playing 61 with the relic build. Mirage Mirror is the real deal, but I would be more inclined to just play 60 in a build with mirror, thinking either going back to 2 mazes or 2 mirrors since the other cards are pretty core to the deck. The core of this deck feels very good though, these are more marginal number changes if anything)
Ensnaring Bridge vs All Is Dust is meta dependent. All is dust is better vs fair decks, ensnaring bridge is better vs unfair/stompy decks.
Relic of Progenitus is very good, fights Deathrite Shaman, mainboard graveyard hate helps immensely vs some matchups like Storm or BR reanimator, significantly slows down fatties like Knight of the Reliquary or Gurmag Angler, helps play around Surgical Extractions that everyone and their mom seems to be boarding lately, and most importantly cantrips so it's never truly dead. Highly recommend.
Mirage Mirror is the real deal! There are so many unsuspecting uses for it and it's such an amazing multi-purpose card! You can even go crazy and stack activations to net an insane amount of mana given correct board state, such as copying candelabra, holding priority and copying Cloudpost.
The most obvious use is that it can copy CLoudpost, but it can also copy Maze of Ith, copy opposing Dark Depths, copy opposing creatures to block, copy
Leovold, Emissary of Trest
to deny draws (great with your own maze or opposing brainstorms!), good response to Show and Tell, etc.
Ancient Tomb in sideboard is to be used with Crop Rotation so turn 1
Sphere of Resistance
or
Warping Wail
can be played vs combo decks. Currently trying out Nephalia Academy since I find cases where you want to turn 1 play 2 cost spell is somewhat fringe, and against decks where it matters I feel you're going to get more out of cropping out Nephalia Academy where it doesn't diminish in power nearly as fast.
