When you choose your spells, the best thing to remember is that you ideally want to be learning the highest possible level of spell you can. Each time you gain access to a new level of spell slot, try to learn 2 spells of that level, unless you really have a specific plan in mind. Keeping your spells varied gives you the most chance to be valuable but the beauty of most D&D classes is that you are free to pick things you think are cool or create your own specific strategy.
For our example build, I've tried to focus on things that I feel match our Chronurgy Magic flavour, which is going to involve choosing spells that give us mobility and control the battlefield by paralyzing or incapacitating our enemies.
When you reach your Ability Score Improvements for a Chronurgist, I would recommend maxing your Intelligence as soon as you can as it's the driving force behind all your cool spells and class abilities. After that, beef up your Constitution and your Dexterity, unless you want to try to make room for some Feats. The Lucky Feat is always good, Tough will help you to stay alive, War Caster will help you to keep your spells in play for longer if you're taking damage and Alert will help to make sure you are always on top of the Initiative order.
So, without further ado, let's take a look at my example Chronurgist's path to Level 20.
At Level 2 I pick up the Chronurgy Magic Arcane Tradition, which gives me Temporal Awareness and Chronal Shift which I described back in the subclass overview. I'm taking Comprehend Languages and Identify, ritual spells that will be useful to me, and the rest of the party, in researching and tinkering.
At Level 3, I get to choose some 2nd level spells. First up, I'm definitely taking Hold Person, which is like a spoiler for the Momentary Stasis class ability I'll be picking up later. I'm also going to take Misty Step, which will help to keep me from getting trapped in close combat, and can really easily be re-flavoured as temporal shenanigans.
At Level 4 it's time to get controversial. I'm going to cash in my Ability Score improvement for the Keen Mind Feat, which takes my Intelligence score up to 18, pushing me into +4 modifier territory. It also allows me to know how many hours away from sunset we are, which seems like a particularly cool thing for someone who can tinker with time.
I'm going all in on the reality tampering by taking the Dunamancy spell Fortune's Favour from Explorer's Guide to Wildemount. This gives me or someone else a second bite at an ability check, attack roll or saving throw. Mirror Image is next – creating echoes of myself makes it look like I've become partially unhooked from time, as well adding to my survivability.
Last but not least for this level, I get a new cantrip. I'll go with Fire Bolt so I know I always have a choice of spells that I can cast at range that deal different damage types.
At Level 5, I'm going to immediately grab Counterspell with both hands – denial is something that this subclass specialises in, so being able to zap spells out of existence seems like a great trick to have up my sleeve. I'm also taking Haste – again very flavourful in that I'm speeding up time around the creature I cast it on.
At Level 6, I get the Momentary Stasis feature from my Arcane Tradition. I'm going to add Slow to my spell book, for obvious reasons. I've now got quite a few useful control spells in my book, which gives me plenty of choices for how to manage the battlefield. I'll still be needed to deal some damage every now and again, so I'll take Fireball – you'd be hard pushed to find a Wizard that doesn't have this somewhere in their arsenal as it's one of the most effective ways in the game to deal with being attacked by large mobs of low CR creatures.
At Level 7, we get access to 4th level spells. At risk of annoying the munchkins, I'm going to take my second pick as Life Transference, a 3rd level spell which will let me do a smidge of healing if it's needed. It can easily be flavoured as a gift of years from the caster to the receiver. I'm also picking Dimension Door, which is one of the best spells you could ever want for mobility and self-preservation (and up to one party member of the same size, so let's hope some of the other players in the party are small too!)
Level 8 brings another Ability Score Improvement. This will be a straight up INT boost, so we're now at the max of 20 with a +5 modifier. This gives us the best chance for our control spells to work. We also get two more 4th level spells, this time Fabricate (because I am an inventor and tinkerer, so this seems to fit that kind of theme) and Banishment (moar control!).
Level 9 sees us travel one rung further up the spellcasting ladder into 5th level spells. I'm immediately taking Temporal Shunt from the new Dunamancy list so I can protect myself and other members of my party not just from attacks but potentially from the villain du jour's plans as a whole. I'll back it up with Hold Monster. Now I can freeze pretty much anything I want!
Level 10 brings Arcane Abeyance, a class feature that takes the Life Transference spell we've already picked up to another level. Using Arcane Abeyance to cast Life Transference into a spell bead that a party member can use later, I can engineer a scenario where I sacrifice life to heal them a greater amount than I gave up, then they can do the same in return. Assuming we both roll OK, we can then effectively have two non-healer characters taking one round to heal each other. Failing that, I can just bung a Fireball in there and whoever gets the spell bead can really surprise the enemy!
For my spell picks this time, I'm going for Animate Objects to hit my tinkerer notes once again, and Scrying – functional and useful outside of combat. I get a new cantrip too, and I'm going with Toll the Dead this time, just for pure flavour.
Level 11 brings us more spell picks at the heady heights of 6th level. I'm choosing Disintegrate because what's a better use of temporal magic than immediately ageing something so much it crumbles to dust? I'm also choosing True Sight – another utility spell that can be used outside of combat if the party gets stuck.
I'm in territory now where I can get spells in my book faster than I pick up new slots to accommodate them – I see myself using True Sight less often than Disintegrate probably, and because I only have a single 6th level slot to spend, it's OK to have a more niche support spell for those just-in-case occasions.
At Level 12 I'm going to be boring. You heard me, it's my right to do it! My ASI will be spent on War Caster (OK, that's not boring, but the next bit could be considered so). For spell choices, I'm looking at things I didn't pick up yet from lower levels. I'm taking False Life to ensure that I can survive for longer (I can cast this into my spell beads if I want) and I'm also taking Remove Curse because we're in Tier 3 now and that potentially comes with creatures that like to leave a mark on you after they're dead.
Level 13 opens the door to 7th level spells, and those will be Simulacrum and Plane Shift. Simulacrum lets my temporal mage be in two places at once, so it could not be any more flavourful, and Dunamancy is about altering reality, so Plane Shift seems like a fair pick!
At Level 14 we get Convergent Future, our final subclass feature that will give us the potential to annoy the DM to no end (good thing we have some defensive spells). Spells-wise, I'm going to take Mordenkainen's Magnificent Mansion, even though it might end up competing for high level slots. It ticks the reality altering box, and it's also a nice nod to Critical Role, without which we wouldn't have this subclass. I'm also going all the way back to the peewee spell levels and taking Cause Fear, which I will flavour with showing creatures mental images of their own moment of death.
Level 15 gives me access to Reality Break from the Dunamancy list, which I am taking immediately (you get to roll on a table of very nasty damaging things to do to your enemies), as well as Power Word: Stun – we're now all in on the time freezing flavour! This time, we have a spell that can keep something out of the game for as long as it keeps failing Constitution saves. This particular spell doesn't require concentration and I can use my class features to mess with its rolls to save, and help to lock it down.
At Level 16 it's ASI time again. I've been greedy on the Feats, so I'm going with a straight-up +2 boost to CON, which comes with a lovely slice of Hit Points. We're back to colouring in spare pages in our spell book, so we'll do that with Lightning Bolt (another non-concentration spell that damages well and scales with upcasting) and Otiluke's Resilient Sphere.
At Level 17 I'm immediately hitting the Dunamancy list again and taking Time Ravage, because it's badass, and then I'm taking Wish, and I don't even care that it's greedy! Nothing alters reality like Wish and the amount of flexibility it gives you is bonkers, never mind that it lets you be a healer if you want!
Level 18 brings Spell Mastery, and so I need to choose a 1st level and a 2nd level spell to be able to cast without spending a slot. I'll take Shield so that I can basically run with an Armour Class of 20 from this point forward, and Hold Person. I also get to choose even more spells for the book, and those will be Sending and Greater Invisibility, both for the sake of utility.
Level 19 is my final Ability Score Improvement, which I'll use to max out my CON, and I can take advantage of my extra 6th level spell slot to widen my options here a little with Flesh to Stone and Mental Prison (more control along the same flavoury lines as Cause Fear).
Level 20 and my final bonuses will be the Etherealness spell to take advantage of my shiny new extra 7th level slot, as well as Teleport. My Wizard class feature Signature Spells kicks in, so I'll take Counterspell and Life Transference (I could choose Fireball but I like being able to upcast it!).