I've been brainstorming Dredge for the past few months to come up with alternatives to both Dark Depths and traditional rainbow land + hate destruction package.
I'd like to present some of my ideas for feedback. I've done this piecemeal in the Dredge discord, but I'd like to have a dedicated forum post for the project as well.
My hope is that people will help identify the more promising candidates, test them, and optimize them. I think even if results are negative we should make note of its applications to some potential future metagame. Finally, I welcome suggestions, especially if the power level of the suggested package or combination appears well-suited to the Vintage format.
The following post-board strategies I regard as having proven themselves over time, or at least having positive tournament results constituting a proof-of-concept. Substantial optimization may be needed.
Anti-Hate Dredge - The classic. People continue to run it successfully, so it's clearly a reasonable place to be. Main variation in my view is whether you are building to function on 0-1 mana (preferably one) or you are building to function on 1-2 mana (preferably two). The former gets you Spinning Darkness and Nature's Claim, the latter gets you things like Cabal Pit and Abrupt Decay to go with that Nature's Claim.
Dark Depths - Clearly a little meta-dependent and somewhat hosed at the moment by Karakas (due to Lavinia). However, maybe we can run a Wasteland package in the main? Karakas doesn't work until after the token is made, after all.
Discount Monsters - Dredge a little, and cast Molderhulk, Ghoultree, and potentially others like Nemesis of Mortals or of course now-classic and somewhat proven fatty Gurmag Angler. Again, a solution best adapted to fight Grafdigger's Cage, and not ideal for the current Leyline meta. Similar considerations apply to other graveyard-synergistic fatties like Tarmogoyf, Boneyard Wurm, Elephant Resurgence, and so on. Special mention should be made of the discount monsters like Hollow One, Phyrexian Dreadnought, Lupine Protoype, Metalwork Colossus, Demon of Death's Gate, and Salvage Titan which have some synergy with the deck but can potentially (in the right build) be played without any graveyard activity. I regard Molderhulk, Hollow One, and Gurmag Angler as proven successful and all the others as speculative (yes even Tarmogoyf).
Fish - Lance Ballester (@vaughnbros) has had success with strategies built around paying 2-3 mana for powerful creatures that are not reliant on the graveyard to function. Of particular note are Mayor of Avabruck as well as a combination of Divining Witch and Laboratory Maniac.
XXX - Run a bunch of X-mana X/X creatures like Endless One and Walking Ballista. Dredge for a couple turns despite your opponent's Grafdigger's Cage or Containment Priest, cast 3ish 0/0s and make 6ish 2/2 tokens from Bridge from Below. I ran it succesfully albeit briefly so it definitely can be done, but it requires a meta of Grafdigger's Cage and Containment Priest, not the current Leyline meta. The strategy is similar in my mind to Empty the Warrens in a context like RG Belcher.
Workshops-Dredge Hybrid - The hybrid deck from @Zias is to my mind one of the most creative uses of the Dredge card pool, but not quite a true Dredge deck, as it is not really reliant on the Dredge cards to put the deck in an advantageous position. To me this reads as a Shops deck that cut Moxen and creatures for the Dredge package, not a Dredge deck that includes a bunch of Ancient Tombs. Note that there is an actual reason for a deck like this to exist, which is that multiple Hollow One can be put into play from a single land tap through a Sphere of Resistance, something unavailable to traditional Workshops creatures. The triggers-based creatures of the Dredge graveyard package similarly function well through your own Spheres of Resistance or even Trinisphere. This helps reduce some risks Shops players face such as when an opponent is able to Wasteland you and trap you under your own Trinisphere.
While all of the above have positive tournament results, what remains are unproven ideas
Dredge-Workshops Hybrid - I put this first solely to say that while cutting the creature package of the Shops deck for a Dredge package seems to work, cutting the spare Dredge material for a list of all the artifact cards a normal Dredge deck wishes it had, like Null Rod and Winter Orb, with the intention of sideboarding Ratchet Bombs and Powder Kegs as your anti-hate, does not appear to work well. So while I was searching the same galaxy as as @Zias in terms of combining Dredge and Shops, I was definitely not in the same ballpark. Still, the Mishra's Workshop / Ancient Tomb package opens up the ability to explore 2-mana artifact cards as potential disruption or anti-hate. That portion of the card pool will be open to us as long as the lands are unrestricted, so perhaps the right cards will be printed or the meta will move in a direction that justifies this kind of plan.
Ravager Aggro sideboard - The reversal of @Zias's operation. Take the head off the Frankenstein and put it back on the body. The proposal is to run Shops + Dredge maindeck, and Shops + Shops post-board. After all, we know Shops frequently wins its post-board games with exactly that plan. The plan might include Lupine Prototype over Hangarback Walker, depending on the space allowed for Shops creatures. 15 slots is somewhat light, I would probably put Phyrexian Revoker and perhaps Arcbound Ravager maindeck.
Colorless Eldrazi - You don't get all the Pitch disruption, but some Eldrazi are synergistic pre-board, such as Endless One which can make extra Bridge from Below tokens or Eldrazi Mimic which can help amplify your Dread Return on something like Emrakul, the Promised End, which is in turn synergistic with the Dredge plan, being potentially castable from hand as a late-game play (perhaps as low as 6 mana off of 3 non-Bazaar lands). Post-board you bring in more expensive Eldrazi like Eternal Scourge (which is synergistic with Serum Powder) and Thought-Knot Seer. Unfortunately Colorless Eldrazi is fundamentally a 4-mana deck, which is more than the 2 of the Ravager Aggro sideboard, so this is probably a worse deck than the one above. Nonetheless, an intriguing possibility.
2-Card Monte If you happen to be running Mishra's Workshops and Ancient Tombs in the maindeck, perhaps this could work. Unfortunately initial testing appears to be negative.
Oath of Druids - My latest inkling. Take a Dredge deck that runs relatively few creatures maindeck, like the Flayer Dredge list @Montolio (Andy Markiton) was running, and get under 15 creatures outside of the Dread Return targets, which will become your Oath fatties post-board. (For this reason I would cut Flayer from the Flayer Dredge deck). Run Crippling Fatique to mill to Oath and clear out Containment Priest, potentially exiling your first target and trying again after removing Priest, as well as something like Laboratory Maniac or Auriok Salvagers to beat Grafdigger's Cage. I believe Tezzerator has run a transformational sideboard into Oath in the past, so perhaps this should go in the proven section, but that wasn't a Dredge deck. Tezzerator had mana production, disruption, and card manipulation in the main deck, which obviously will not be included as fully in a deck whose shell requires dredgers rather than lands to operate.
Uncounterable Anti-Hate - I have a thread up on this, regarding Evoke creatures and Cavern of Souls. I have been told that Mark Hornung has tried this unsuccesfully, but perhaps a heavy dose of maindeck disruption would make it work.
Unwasteable Anti-Hate - You can potentially have a Wasteland-proof manabase for your sideboard removal spells simply by running basic land and a single color of mana (or Flagstones of Trokair for white in particular). Thanks to Spinning Darkness and Dismember breaking the color pie, you have a real choice of Blue, White, or Green plans with cards like Chain of Vapor, Void Snare, Kiora's Dismissal, Nature's Claim, Emerald Charm, Natural State, Fragmentize, Wispmare, and/or Erase. The White and Green plans are somewhat more enticing against non-Leyline hate due to the fact that Riftstone Portal operates through most graveyard hate. In theory red perhaps could be run with Enchanter's Bane.
Fearless - I think perhaps Pitch Dredge has enough countermagic and discard that you could in theory go Fearless and just bring in a few cards to optimize your deck versus different matchups (you know, like a normal Vintage deck). This type of build benefits from Chancellor of the Annex as the finisher (to sometimes delay hate from the opening hand) and Hollow One as a beater in case you don't get there on countering the hate. Extremely soft to Leyline, but perhaps favorable enough vs other hate.
Phyrexian Dreadnought - I mentioned him as a speculative fatty but I want to single him out here as being particularly easy to set up (just play Torpor Orb and/or Illusionary Mask) and consistently castable. Additionally combos with Eldrazi Mimic as well, although that complicates the mana base.
Desecrated Tomb - Not functional against Leyline, but spits out 1/1s vs any other hate. Each Dredge and Narco/Prized Amalgam/Bloodghast trigger results in a 1/1 except against Cage, and each Ichorid trigger can generate 1/1s even through Cage. Any exile sweepers like Tormod's Crypt will reward a 1/1, which is not tremendous but if it's the 4th or 5th token perhaps it makes a difference. Worth noting that 3 mana is kind of a stretch to hit quickly for an Ancient Tomb deck with no Moxen and Bazaar of Baghdad using your land drops, even if you do have the Mishra's Workshops to sometimes get there.
That's all I can think of for now, please take a look and let me know if you think any of them are particularly promising. I get that most Magic ideas don't pan out, so if I don't hear anything about a particular possibility I will assume the mood is somewhat negative towards that option.