Opt has been one of the more hyped cards from Ixalan, and for good reason. We haven't seen such an efficient cantrip at one mana since the days Preordain and Ponder coexisted in Standard, and we as a community collectively failed to appreciate such a bounty.
There are clearly going to be implications from the reprint in Standard, but also in Modern, despite the existence of quality cantrips in that format. There are certain decks that want a critical mass of cantrips that will be bolstered by the addition of Opt to the format, and as a devotee of cheap spells I want to find those decks.
The obvious landing point is Storm, a deck that has experienced a resurgence in recent months due to the printing of Baral, Chief of Compliance. More velocity is always welcome in such a deck, but let's go beyond the obvious to something more aggressive, more my style.
Prowess decks have lots of tools in Modern and don't leverage Thought Scour's self-milling nearly as well as a deck with Past in Flames, so the upgrade to Opt is even more pronounced there. Thing in the Ice and Kiln Fiend are powerful threats, but they need you to be able to consistently produce multiple-spell turns rather than set up for one game-ending turn, so your velocity is quite important.
With Opt, you can often tap out for a threat or two on the first couple of turns of the game and cast three or four spells on Turn 3. This can end the game outright if things go very well or set you up to repeat the process on Turn 4 to end the game. That kind of sequencing gives you the speed to compete in Modern, and your velocity provides the consistency.
There are a lot of ways to build this deck with various prowess threats available, but the key is having the density of quality spells to fill out the rest of the deck. Opt is more powerful than it looks and makes spell-focused decks like this one a strong option in Modern going forward.