I agree that we can put the Deadly++ idea to bed. And I also think that it is definitely important to make the encounter dynamic and interesting in addition to difficult.
In the past I have made some really bad nova battles -- lone BBEG, static environment, not considering the full capabilities of high level casters, etc... One of the reasons in starting the thread was finding out if it was me (spoiler: it was) or if it just wasn't a worthwhile form (jury is still out).
Plus I have learned something I did not consider before: how short and long rest classes differ in their potential output when novaing. It's worth digging into.
So my experiences, with the various classes and NOVA capability (there will be holes, since my group heavily gravitates toward certain classes):
Artificer - A hole for me right of the bat (have seen exactly 1 artificer played in 5e, no one took one even in my Eberron campaign): But it's a half-caster with a lot of magic item related powers. Does not SEEM to be a big NOVA class, more of a long haul class. But, I'll leave specifics to someone more familiar.
Barbarian - Not a big NOVA class. BUT with this paradigm rage will be active nearly 100% of the time for any given fight (unless there are conditions that make it suboptimal). This means putting the Barbarian down will be very difficult. That's about the only (but big) NOVA issue though.
Bard - A full caster with some serious oomph, if they want. going NOVA means no regard for holding back - so expect Bardic inspiration every round. Expect serious spells like sleep, Fairie fire (low level), Hypnotic Pattern, Fear (mid levels), forcecage, charm/dominate monster etc. (high levels) and other encounter stopping spells.
Cleric - Full caster again. The big thing with the cleric going full bore every encounter? Healing word means, even IF someone drops, they won't stay dropped -and since the cleric isn't conserving slots this will just keep happening. Since it's got an ok range, the cleric can even do this from the back line. AND can do it and still attack from range (with a cantrip, but still). All the while they will have spirit guardians etc. to make enemies regret closing. If you have a twilight or a peace cleric and your group is only expecting the one encounter? at mid+ levels, you will not be able to keep a party member down without ludicrous damage and/or effects.
Druid - Another full caster. And has healing word and other spells to keep the party up just fine. At low levels, Moon Druids will LOVE being able to morph into bears etc. with no regard/worry as to saving shapes (though you CAN make them regret it with a nice exploration challenge later). Not quite the oomph of the cleric, but near enough to make anyone in the party going and staying down a fairly difficult endeavor.
Fighter - Not a NOVA friendly class. Except, expect the fighter to action surge in every encounter. And since he will be buffed and attacking with advantage (even at level one with things like Fairie fire) this will do a ton of damage and needs to be accounted for. The BM fighter will be using a maneuver die near every round (why not, most fights take 4ish rounds, they get 4 dice) which will increase their output a lot. But the impact will, almost certainly be less dramatic than the casters. Especially as levels rise.\
Monk - The monks primary worry is always running out of Ki. With this paradigm, at all but the low levels, that's less a concern. So at 5+ expect multiple stunning strikes on any given round - this means solitary monsters are done for, unless you pile on the legendary resistances. And even then, the monk is softening them up for the caster. Still, I don't see too much challenge designing around this as necessary.
Paladin - Half caster, but with smite, So they NOVA quite well. Smites every round will crank up the damage and the paladin aura (for 6+ level) means save effects are lessened. Frankly a pretty good martial to have in the party if this paradigm is expected. Plus, spamming lay on hands means little worry of after effects of the fight (though usually suboptimal during the fight).
Ranger - half caster. Unlike the others (except for maybe monk) this one will be a bit subclass dependent. Gllomstalkers in particular will thrive as that initial oomph (the first round) can change the flow from moment one (plus they can push a dramatic initiative bonus). I can just see a bugbear gloomstalker ranger make even the paladin player gape at the first round damage. Plus, since other encounters will not be combat but exploration etc. - they can REALLY make a big difference. depends on what there will be outside of combat though.
Rogue - No NOVA at all really other than a bit for the arcane trickster (and a tiny bit on the soulknife). But rogues are pretty solid, so should be able to hang in there. Plus, hopefully serious out of combat utility. Depending on what surrounds the one NOVA encounter, this class may end up being avoided.
Sorcerer - Full caster with lots of NOVA potential. getting to spam sorcery points AND spells means the sorcerer can make this paradigm his wheelhouse and significantly affect every encounter (since they essentially have double nova potential).
Warlock - A "kind of" on the full caster. But because we're expecting only 1 encounter, their small spell output isn't nearly as limiting as it could be. Plus all their lower level spells being at "max" level will help a lot. Could excel here, with the right build. And certainly wouldn't be too far behind otherwise.
Wizard - Hey look, another full caster. Full and huge NOVA potential. A wizard not having to conserve spells is a terrible thing to behold -expect ALL the biggies to make an appearance. So the DM then has to focus on legendary resistance, spell immunities etc.
Well those are my quick takes on the classes under this paradigm.
Thoughts?