I'm skeptical of Brownscale because I really want to maximize consistency, but IMO it's not totally crazy for two reasons: if you have multiple dredgers you can use the higher-valued ones, and when the deck gets low in card count sometimes you do actually want a smaller dredger.
4 Grave-Troll, 3 Hogaak, 4 Force of Vigor, 2 Shambling Shell, 3 Green card X (probably Noxious Revival) gets you to a reasonable green count with pretty minimal constraints on your deck building
Similarly 4 Force of Will 4 Force of Negation 4 Narcomoeba 4 Mental Misstep gets you to a nice blue count
I don't think the whole blue and green count issue requires running a lot of suboptimal cards just to pad the color count
Edit: I think the bigger disagreement is regarding which cards are 'sacred cows' that don't actually contribute all that much to wins versus which are impactful. @vaughnbros with 12 dredgers, 4 Ichorid, 4 Narco, 4 Bridge, 4 Therapy, 3 Hogaak, are there any of those slots you would replace with non-graveyard effects? It looks to me like you are cutting "sacred cow" cards for other cards which fulfill similar functions, in which case it would seem that you need some density of graveyard effects and the question is simply which is best.
The green cards is something to work out. Noxious Revival seems ok, but not great. There are some ok, but not great Dread Return targets too. 4 Grave-Troll, 4 FoV, 2 Hogaak is the starting point 10 that I think should be played. Finding another 6 good cards is a task that I think is easiest to just throw some Dredgers into, but there are probably better options. Dryad Arbor is one that I'd really like to get working as it was an old Extended staple of Dredge decks ~10 years ago.
I am pretty adamant that I think Bridge from Below is straight garbage in a Dread Return based list. I've been playing without it / minimal copies for like 7 years now. Some how this card got Sacred Cow status, but I don't think it was well deserved. Triskelion and Walking Ballista's rise to power really made it not good in any real matchup of importance, and if you hate Amalgam for its win more qualities than you should really hate Bridge from Below. Without Chewers and Wispmares, it makes no sense to run in these lists. If you cut Dread Return instead, then sure, it has some applications on overcoming difficult board states. I think its really Dread Return OR Bridge from Below, but not both. If we think that Return is part of the core then I wouldn't play Bridge, if we think that Bridge is part of the core then I wouldn't play Return.
Mental Misstep is not worth slots. I've also been playing that on-off again since it was printed. In a format where Grafdigger's is not a primary hate piece of any deck, I just don't see the added value. I tried it a little bit during these 35 matches, and it just pitched to Force every game or sat in my hand doing nothing. It makes more sense to slot in graveyard cards there. I've like Amalgam, but might start looking into Fatestitcher and Chain of Vapors out of the board again as I think having no mana in the deck is a sketchy situation against Shops/Outcome builds.
Therapy. I kind of covered this above, but I just don't see the matchups where it helps. Mostly has been used to clear countermagic from control decks that we are heavily favored against, even if they Force my Dread Return, I've easily been able to recover and still win in a timely fashion. If we are cutting Dread Return it serves no purpose of clearing countermagic. You need to hit into multiple copies to really impact any combo deck. Does almost nothing against Shops/Survival. Ok against the mirror to exile their Bridges. Really just feels better served elsewhere. I think I'd rather have Ancient Grudges in these slots, or something else that can boost Combo matchups.
Ichorid is good, but replace-able by either Amalgam, Nether Shadow, or a critical mass of free creatures from your hand. These turn 3 creatures are really just there to provide some consistency on top of the Narc and Ghast. I've run lists that just have Narc+Ghast before, and was quite honestly fine for the most part. You run into the random game where you exile too many of them, but that was few and far between.
The core required for winning is much more concise at:
Some number of Blue returnable creatures (e.g. Amalgam, Fatestitcher)
Some number of "free" creatures (e.g. Hollow One, Rootwalla, Dryad Arbor)
No superfluous copies of Therapy.
Or the new route of:
Bridge from Below
Maybe some "free" green creatures (e.g. Rootwalla, Dryad Arbor)
Again, no superfluous copies of therapy.
These configurations potentially have more room to support cards like Lion's Eye Diamond and Black Lotus into Ancient Grudge / Fatestitcher / other powerful flashback spells.
I've been banging down the first route with Dread Return, and some of my fellow Academy members have been going down the second route, one of those 5-0 lists that cut return.
The list you linked has 49 cards in place of the 37 I'm running for the core dredge plan
Even if you don't count Hollow One it's 45 vs 37
My OG pitch lists had 42-43, some of which mattered without the graveyard (for reference)
Let's actually map this out, if the claim is that we want to cut unneeded graveyard stuff to fit in more real cards I'm 100% down with that but we should actually be confident we are accomplishing that goal.
OG Pitch (43 slots):
4 Serum Powder
4 Chancellor of the Annex
3 Dread Return
AJ's minimal core (37 slots):
2 Serum Powder (post-London, could go up to 4)
Do you have a list that takes up less than 37 / 39 (depending on the Powder situation) for the dedicated graveyard stuff? I know you don't want to overly focus on it but the list in your OP has 44 slots of graveyard stuff (not counting Riftstone Portal).
Here's the diophan list you linked (49 slots):
4 Hollow One
Pre-Amalgam days, I had lists that were more concise on number. I think you are right that this is probably the card causing dead weight.
List from 3 years ago that has 49, but was a fairly consistent turn 2, and has a decent main deck manabase: https://www.mtgtop8.com/event?e=11771&d=267126&f=VI
Could be cleaned up a bit with a smaller return package that's not using 6 slots on dread return targets.
This slightly earlier incarnation had 41: https://www.mtgtop8.com/event?e=11679&d=266568&f=VI
And could also easily be cleaned up below that without double bridge, and 5 slots on dread return targets.
Killing people with Hermit Druid post board were the days.
Let's say we want to make a dedicated Dread Return deck, which seems to be one of the routes you're considering.
X Dredgers (maybe 12, maybe more or less - hard to say when you have to run Dakmor Salvage)
4 Dread Return
4 Dread Return targets (maybe Chancellor?)
That puts us at 36ish prior to including additional free creatures or speed elements. It might be the way to go but I don't think it's more compact than the traditional configuration with Hogaak in the Dread Return slot. Even if the Hogaak list is 40 (4 Hogaak 4 Powder) this would only save 1 4-of slot and you wouldn't get to include any additional speed elements like free creatures or Fatestitcher, or additional consistency elements like further graveyard creatures.
Is that fair?
I think Fatestitcher is a reasonable speed element if you want to be on a fair bit of mana (in the 1, maybe 2 range, like the 2013-2016 dredge decks)
My feeling is that without Bridge from Below you need "bigger" Dread Return targets that can win the game on their lonesome, things more in the line of Iona, Elesh Norn, etc. Chancellor would be nice to give more disruption from the hand but I don't think you can just return a Chancellor and feel confident vs a lot of decks in the format.
My concern is that without Bridge it's harder to chain together multiple Dread Return, but I think it's more something to be ironed out with testing / tweaking than a real objection to the whole archetype.
So I think this whole no-Bridge DR thing is workable, but I don't see it saving slots for more disruption.
Is there another list I should be thinking about that saves more slots?
2 Serum Powder (post-London, could go up to 4)
Don't you just cut Therapy from this and play Bloodghast over Narcomoeba? This would require you to find ways to interact as the deck will be slightly slower, but you can easily generate lethal on turn 3 or 4.
I value Therapy much more highly than you do, Bloodghast much more lowly, and I think Therapy + Narco is a generally faster way of accomplishing the same thing as Bloodghast in that configuration.
The other issue is Dakmor Salvage, which is either taking up more slots or decreasing your average dredge (or both).
I think the Bloodghast/Ichorid/Hogaak/Bridge configuration would be a fine thing to test but I'm not particularly drawn to it.
Edit: Can we map out the other alternative? I think it was cutting Dread Return from a traditional package
Edit 2: Just to clarify, when I say I value Therapy more highly I'm saying I think it has a disruptive effect from the graveyard as well as being a sacrifice outlet from the graveyard. This makes me reluctant to cut it for a disruptive card from the hand which has no particular graveyard synergy.