The Locust God - $7.99
Six-mana finishers that require two colors to cast are not my favorite spec targets. There's already a U/R Control deck in the format, but I feel like The Locust God is worse than Torrential Gearhulk most of the time in that build. It might be a solid sideboard card in a few different control variants, and I know that the New Perspectives mages are stoked to try it, but I can't see it becoming ubiquitous. The current retail price is totally fine if you're buying a few copies for yourself, but The Locust God is a longshot to become the next great format staple.
Neheb, the Eternal - $4.99
My first reaction was to completely dismiss Neheb. How many red five-drops do we need? Glorybringer appears to have more raw power, and this even competes for space with The Scorpion God. The competitive community seems to agree with this take, and the safest thing for me to do is to label it a future bulk mythic.
But holy cow is this card good if you're able to deal any combat damage. Drop it in your precombat main phase, attack with something else, deal some damage, and use that mana to cast another threat. It's a way to turn a battlefield when you're either a little bit ahead or in a racing situation into an overwhelming advantage. "Free" spells always get Wizards into trouble, and it's possible that we're looking at something similar here.
I'll be honest—it's a longshot. The safe play is to stay away. But these are the cards where you look back on a few months down the line and wonder why we missed it. As a $5 mythic, there's easy double-digit upside. Color me intrigued.
Mirage Mirror - $3.99
I want to clarify two points before we evaluate this bad boy. First, Mirage Mirror does work with Dark Depths. It remains to be seen if that will help boost the Legacy deck at all, but I wouldn't be shocked if both Dark Depths and Thespian's Stage see small bumps in the meantime.
Second, Mirage Mirror doesn't work with Thing in the Ice. If you turn it into Thing in the Ice, it would have zero counters on it, need you to cast an instant or sorcery, try to flip when you do, and fail to transform. If you turn it into Awoken Horror, the mass bounce won't trigger. Sorry, friends!
Moving on, Mirage Mirror is the sort of card that will be a Commander staple approximately forever. That gives it a very high floor. There's a decent shot it ends up in some sort of bizarre Eternal combo deck as well. I don't know if it's good enough for Standard—probably not—but the price floor is so high that I'm willing to buy a few copies just in case. Worst-case, I can't see Mirage Mirror going below $2 for long. Best-case, it's one of the most powerful and versatile cards in the set.
Solemnity - $2.99
Wow, what a powerful card. In Standard, it shuts down Walking Ballista while making it very, very hard to play most of the green decks—G/B Energy, Temur Energy, B/G Delirium, R/G Energy…basically, everything with Winding Constrictor or Verdurous Gearhulk. Most of those decks have excellent enchantment removal, but I still bet Solemnity sees extensive sideboard play.
In Modern, Solemnity does double-duty, stopping both Infect and the Devoted Druid Company decks. It's no Stony Silence, but it's probably worth sideboarding in against Affinity, too—and in a format where you need maximum versatility out of your sideboard, that's big. In terms of proactive strategies, it's worth noting that this enables Kitchen Finks combos while making you even more immortal with Phyrexian Unlife. I see implications as far back as Legacy, where this is yet another solid part of a Dark Depths strategy. I've even heard rumblings about a Glacial Chasm combo in that deck, since Solemnity negates cumulative upkeep.
At $3, it's silly not to grab a set. Solemnity seems destined to affect multiple formats, and you'll want to get your copies ASAP. This will probably settle in the $5-$6 range short-term until we see how good it is. I bet there will be a series of related spikes as well, with Phyrexian Unlife, Glacial Chasm, and Dark Depths all gaining value in the coming hours.