Unmarked Grave from Modern Horizons 2 does a great impression of Entomb in a way that is Modern legal!
The most common reanimator targets in Legacy are often Legendary creatures such as Griselbrand or Iona, Shield of Emeria which are out. But there are still plenty of ways we can go with Unmarked Grave to achieve a similar power level.
Footsteps of the Goryo into Protean Hulk might be the easiest way to just curve into a turn 3 win. Protean Hulk finding Disciple of the Vault and a bunch of Walking Ballista and Hangarback Walker is a perfectly legal combo in Modern, it has just been short on enablers up until this point. Typically the biggest downside to Hulk combos is that you have to dedicate a bunch of slots in your deck to combo pieces, but these particular 0/0 artifact creatures are good cards in their own right and this combo is just incredibly easy to assemble.
Unmarked Grave can also find a large number of cards that you'd want in your graveyard besides just a fatty to reanimate. Life from the Loam decks typically function much much better when they find their signature card. Unmarked Grave could also be a piece that completes the puzzle for a traditional style reanimator deck, letting us find Unburial Rites if we already have a fatty to target, or perhaps just a piece to fuel some sort of Vesperlark / Body Double combo.
While considering cards with Flashback to try pairing with Unmarked Grave, we remembered Echo of Eons. Could this archetype be getting just enough support to do something relevant in Modern?