Most of the creature base that makes Dredge a deck in Modern is legal in Pioneer, so we just need to find a good way to fill our graveyard despite lacking the namesake mechanic. While Treasure Cruise and Dig Through Time are still legal, we should take advantage of the blue self-mill package.
Self-Mill Delve Decklist (Pioneer)
4 Drowned Secrets 4 Treasure Cruise 2 Dig Through Time 4 Merfolk Secretkeeper/Venture Deeper 4 Tome Scour 4 Opt 2 Haunted Dead 4 Arclight Phoenix 4 Creeping Chill 4 Narcomoeba 4 Prized Amalgam 4 Watery Grave 2 Choked Estuary 2 Drowned Catacomb 12 Island (335)
Treasure Cruise and Dig Through Time were quickly banned in every format they were previously legal in for good reason. With fetchlands banned in Pioneer most decks won't be able to take advantage of them as easily, but we're going all in on a self mill strategy that takes advantage of the graveyard, so these will often cost just one or two mana.
We have the enablers to "go off" without Drowned Secrets in play, but games are just so much easier when you cast this on turn two. Building around this engine means we won't be the fastest combo deck in the format, but we'll be consistent.
Merfolk Secretkeeper and Tome Scour do a great job at fueling our delve engine even when we don't have Drowned Secrets. When we have both, we can quickly flip most of our deck face up. They also happen to both be 1-cmc spells to trigger Arclight Phoenix. We can afford to play these card-disadvantage synergistic cards when are easily recouping the value whenever we cast one of our delve spells.
We have more than enough options for creatures and value to get for free from the graveyard when we are milling ourselves. Arclight Phoenix is the most difficult to return, but with our chosen blue-based engine it is reliable. Narcomoeba and Prized Amalgam just need to get in a chip shot or two before the damage from Arclight Phoenix and Creeping Chill gets our opponent. All of these free creatures and lifegain can also help us stay alive against an aggressive matchup while we are spinning our wheels.
Haunted Dead is actually a deceptively important piece of the engine. We are playing quite a few "dead" cards that we don't want to draw and in fact are much more powerful if we have a way to put them in the graveyard. Haunted Dead is the most efficient from a mana perspective when compared to jump-start cards like Radical Idea. It also supplements Narcomoeba in providing an entirely graveyard-based way to return Prized Amalgam to the battlefield if we are temporarily stuck on our Arclight Phoenix plan.
There are plenty of options to consider, and the exact numbers of each of the delicate pieces in this deck will require further testing to determine. We need to find the right mix of discard outlets, delve spells, and cheap spells to trigger Arclight Phoenix.
Considering the fact that cards like Rest in Peace exist, we may want to utilize a transformational sideboard plan. If our opponent is slow enough and is expected to have sufficient graveyard hate, we could relatively easily transform into a mill deck. However, it's worth keeping in mind the fact that our delve spells are turned off by Rest in Peace and Leyline of the Void, so perhaps we are better off just having a plan to deal with artifact and enchantment based hate cards.