Wow, Nexus is strong. I think it can quite reliably combo by around turn 6-7 now. If it draws Growth Spiral, this can drop by a turn. The 3-4 mana plays - Narset & Tamiyo, not to mention Drawn from Dreams - add a huge amount of consistency. Tamiyo even hits 6 loyalty the turn she comes down if you tick her up; that's not easy to kill on turn 4. I haven't felt the need to change anything maindeck from the GP Denver list. The "missing" cards from the mainboard are Opt (it's always been filler, and Narset/Drawn from Dreams are just so damn good at finding what I need), and Nissa (which I have not tried, but do not understand: doesn't she just turn on your opponent's creature removal? You aren't usually mana constrained either if you hit your land drops).
I did change quite a bit in the sideboard though.
Brief how-I-sideboard guide:
InVeil of Summer - vs. any deck relying on blue or black interaction. This can be Thought Erasure, Unmoored Ego, Duress, Dovin's Veto, T3feri, Deputy of Detention ...
Negate - vs. any control deck, and the mirror.
Shifting Ceratops - vs. monoblue, any deck that probably runs few creatures but has planeswalkers to attack (e.g. Jeskai planeswalkers, Scapeshift), whenever I need an alternative win condition (e.g. vs Unmoored Ego).
Aether Gust - vs. decks with the relevant colors, especially anything with big Chandra. It's not a great card, but it can functionally be a Time Walk and sometimes that's all you need.
Biogenic Ooze - whenever I feel the need to change win conditions, and against big Chandra, because in those situations I really want to kill quickly.
Cerulean Drake - against any red deck. It's not going to win you the board, but it can buy you a lot of time (significantly less against Legion Warboss, unfortunately).
OutRoot Snare - against non-aggro decks. This includes the mirror, Scapeshift, Bant Ramp, etc. It's not an all or nothing card though. I can easily see keeping a couple of copies against a semi-aggressive list.
Chemister's Insight - Outside of Root Snare against non-creature decks, it's the first card on the chopping block. It's a testimony to how powerful Nexus's card draw is that this card is now cuttable. I take it out against any control deck, especially decks with Narset.
Narset, Parter of Veils - against aggressive decks. She's not
bad, digging four cards deep and gaining some life is fine, but if you can't get two activations with her often then she's probably not worth it.
Drawn from Dreams - if I still need more cuts (after cutting all Insights) against an aggro deck. In a vacuum I think it's better than Insight. Yes, Insight is an instant, but this card literally gets you anything you need, from Blast Zone to Nexus to Wilderness Reclamation. For example if you have four lands then Reclamation into Insight is great; on the other hand you could also Drawn from Dreams into lands or Nexus, play Reclamation the turn after, and combo.
Callous Dismissal - usually when I switch win conditions. I keep it in against any deck whose main defense against the combo is T3feri. Note
Cerulean Drake is also a win condition against red aggro; you can cut Dismissal then even if you don't add Ceratops/Ooze (which you shouldn't, they're too clunky).
Blink of an Eye - if I feel my opponent's interaction is not worth blinking, e.g. vs. monored, since the best card to bounce then is Experimental Frenzy or Legion Warboss and then it's just not worth it. Blink is essential against any deck with T3feri; it's really hard to combo if T3feri is on the battlefield.
Tamiyo is too powerful to cut in my opinion - aside from having both a flipped Azcanta & Nexus at the same time, she's the best card to untap with. She's good enough that I will often try to defend her with Root Snare the turn I play her.
Random tips:
- The onus to interact is usually on the opponent, not you. Your sideboard is for dodging the opponent's interaction, not to interact with their gameplan.
- Growth Spiral is the best turn 2 play unless you have no more lands, in which case Azcanta is better.
- You tick up with Tamiyo most of the time, but against aggro decks you can conceivably tick down to rebuy Root Snare, fog (keeping Tamiyo alive), and then tick up the turn after looking for Nexus.
- Be ready for people to not concede even in the face of an unstoppable Nexus combo. It's frustrating but what the hell, you're winning not them.
- You must be ready for Unmoored Ego out of Esper decks. They will likely fire it off quickly if they draw it. You beat that card with Veil and Negate. If it resolves and they name Nexus (usually they should) you can also potentially beat it with Biogenic Ooze + Wilderness Reclamation.