TL;DR: I feel like this is the best UR Delver has been since Treasure Cruise.
The biggest hurdle, in my opinion, to running tempo decks successfully in Modern is that we don't really get great card selection or draw power, which really can hamper us in a way that we wouldn't be in Legacy or even Pauper where we can access Ponder and so on. Opt helps, but is pretty limited, and Serum Visions just plain has never been great. The answer, I think, is to just to draw more cards through brute force, and I think Chart a Course and Of One Mind might help enable us, as does the printing of Sprite Dragon, the creature URx Tempo has wanted forever in Modern.
Here I've opted to try and abuse Of One Mind, which gets us an incredible 2 cards for 1 blue mana, conditionally. Luckily, our creature base already gets us most of the way there, since Delver of Secrets
can fill a human or non-human slot, Young Pyromancer can do both, and we'd be likely to play them in a spell-heavy tempo deck anyway, along with some Sprite Dragons and Snapcaster Mages. Mutavault costs one mana, but can enable the draw in a pinch while netting us an extra body.
I'm not sure how well tempo is positioned right this moment, but there's definitely potential here, and new tools I wanna play with, so gonna build some kind of tempo thing to get back into Modern.
I'm far from settled on the sideboard. The one weird choice that may stand out is Blitz of the Thunder-Raptor. I like the exile feature, I like that it scales like Skred. Seems better than when Izzet decks relied on stuff like Roast, Dismember, and Rending Volley, though still inferior to one-mana black removal in Grixis or Dimir decks. Jury's still out. Extra Burst Lightning may just be better. Magmatic Sinkhole is good, but can't hit face, and I see a bunch of 6-toughness guys out there and not many 4-5s.
Also note I've not played Modern in any capacity for a couple years, so I may be missing some key developments. Still figuring out the best plays for Force of Negation, and which decks it's critical against. It is likely to end up in the mainboard, maybe as a 3-of, but haven't actually bought a set yet.
UPDATE: Okay, Force of Negation has made me re-think everything. I was thinking it was a poor man's Force of Will, and it is, but unlike the way that card frequently gets used in Legacy, to disrupt on turn 0 and turn 1, where Force of Negation shines in Modern is that it lets you tap out way more freely, which is huge. The consequences of that are that I question the inclusion of Spell Pierce at all for starters. Next, I have to question Opt, because it's greatest asset is that it's an instant, so a great thing to do with mana you've held up for a counter if it doesn't get used. Problem is, I would want to use those slots for more burn, but then I may not have enough blue cards for FoN to be consistently on tap while I'm tapped out. Gonna mix it up and see how it runs.
Further UPDATE: Got rid of 1-mana, soft, and highly conditional counters altogether. It's FoN or bust, except when I board in Deprive. One note about FoN is that it has definitely biased me in the direction of burn spells that can be pitched to it, like Izzet Charm and Electrolyze. Ionize and Beacon Bolt will be considered as well. I don't think they're out of the question, though I suspect Ionize isn't gonna do the job. In any case, ditching some mono-blue spells and Deprive has made me consider running Blood Moon out of the side.
Even Further UPDATE: Abandoned a lot of the add burn via multicolored spells plan for the moment. Wanted to play with setting up Of One Mind draws better. Went with more Islands anyway to make Blood Moon work, but decided to go very cantrip-heavy and put Deprive back on sideboard. What really hit me was that I should try cutting Snapcaster Mage. If I'm often using him to recur OoM anyway, why not use Mission Briefing to set it up while I'm at it? Plus, it flips Delver and usually nets me a +1/+1 on Sprite Dragon and/or a token anyway, so losing out on the 2/1 is pretty irrelevant. Did pop in an extra Swiftspear while I was at it, to add a human and a T1 threat. This kinda commits me to a strategy where I'm playing some creatures plus Remand, Force of Negation, Lightning Bolt, Vapor Snag, OoM and a bunch of enablers that help me find and recur those spells. I'm therefore gonna see how it runs with fetches, because the deck-thinning seems more relevant.
Last UPDATE: Probably not gonna buy Scalding Tarn right now, but I'm really liking this deck. I feel like this is the best UR Delver has been since Treasure Cruise.
More on Of One Mind: Ok, so this doesn't always always get you 2 cards for , but it usually gets you the two cards plus something you'd want to be doing anyway for or plus when you have it in hand. With Pyro on board, any spell for plus gets you the spell, two cards, and a token for . Any non-human on board plus Delver or Swiftspear gets you a threat and two cards for , plus maybe a prowess trigger. With any creature on the board, Mutavault gets you a 2/2 land body for the turn, and two cards for . All still good deals, especially when casting a spell is often getting you a counter on Sprite Dragon, or a prowess trigger, or a token as well. I'm increasingly convinced by Mission Briefing here. The spell trigger is better than the Snapcaster body, and it sets up very good 2-card draws, or sets itself up by putting something in the yard. Occasionally, it does get stuck in hand a bit, especially if we draw it early, but then it's a good thing to toss for FoN.
Tweaking: Liking my creatures, probably won't change anything. Liking may manabase well enough. Mystic Sanctuary has surprised me at how useful it is. The only things in the deck that I see as flexible are the 2x Burst Lightning, and maybe 1X Opt. Burst could be replaced by another burn spell like Electrolyze, maybe a one-mana counter like Spell Snare or the 4th Serum Visions, same with the 4th Opt. A 4th Remand isn't out of the question either. For now, 2X 1-mana burn that can hit face seems right to me. 4th Opt vs 4th Serum Visions is pretty debatable.
Please comment! Definitely interested in hearing points in comments for and against Blitz, points on sideboarding generally, and points for and against ditching some number of Opts for Burst Lightning or other burn, or other counters. Manabase may be open to debate, but I'm likely not going with fetches or shocks.
