Modern Horizons 3 is bringing some interesting pieces to Spirits. All sideboard pieces but each very potent.
Once upon a time I would see Spirits decks siding Unified Will to counter bombs and wraths. Flare of Denial is just better. I think asking for a Spirit Vodalian Hexcatcher is probably a bit much. But a one shot hard counter variant is still sweet. This can duck a full wrath or a bomb threat or combo piece.
Flare of Fortitude is actually incredibly similar to Flare of Denial but instead of blanking a spell threat, it blanks board threats. I often find when playing aggro, you race your opponent until then sweep your board and you have to rebuild, likely only one more swing from winning the game. With white flare, we can sac a creature to fog an attack and hopefully pur remaining board can push lethal. We can also shrug off Engineered Explosives and the like.
Sideboard tribals spirits tend to miss on fliying, but not White Orchid Phantom. Double white can be a bit pressing but it's still a solid ghost for blowing out a Tron land, Valakut, the Molten Pinnacle, Cabal Coffers or any other problem land.
While I didn't add it, Vexing Bauble looks like a piece of sideboard action (along with Urza's Saga toolbox piece) that is primed for current Modern. The implications in this card are enormous. Blanks evoke, storm, cascade, affinity, forces, flares, and the list continues for every format this thing will be legal in. I have a weakness for trying to keep the sideboard within the theme or brand of the main but certain pieces (like the flares) are simply too good to pass on. With Fury out of the format I'm going to lean towards keeping it off the list for now, but in a meta where those mechanics may be more prevalent this is one of those pieces that you may include even if it is off brand.
