Howlgeist is pretty bad for six mana and I think Doubling Season is just overkill. I also think Gravedigger is just too slow. You should be playing the fourth Elvish Visionary, the fourth Phyrexian Rager, and you should definitely be playing Vengevine. Graveborn Muse should also be in any non-aggro black deck. I also don't see any reason not to run the full set of gates so cut one of each basic land to fit in the other two.
-2 Gravedigger
-2 Doubling Season
-2 Howlgeist
-1 Swamp
-1 Forest
+1 Elvish Visionary
+1 Phyrexian Rager
+2 Graveborn Muse
+1 Vengevine
+2 Golgari Guildgate
That brings you to 60 cards. If you make those changes you get a lot closer to my deck.
For comparison, my Golgari deck makes the changes I recommended and then plays three Pelakka Wurms, Griselbrand, and an Elder of Laurels where yours uses the three Connoisseurs and two Rescues.
Vengevine is a nice addition, no doubt about this, Il'll try to make space for it, thanks.
About the fourth copy of Elvish Visionary/Phyrexian Rager and considering the high rate of rescue from the graveyard, for now I'm pleased with three of each.
I'm a little perplexed with the Graveborn Muse usage because I have very few zombies in this deck but maybe I can try, it's unquestionably a great card, though I have yet many effects in this deck to draw card.
About lands, I had, in my previous version of this deck 4 copies of Guldari Gate but I think that it works quite bad with Satyr Wayfinder and with Elvish Visionary, so I prefer more basic lands.
And, as I said in other posts, I admit that Doubling season is my obsession, but finally I have found a deck in which it is very useful.
Nemesis of Mortals can easily become a tank with one or two Doubling season in play, as well as Bloodflow Connoisseur.
Therefore, playing a Spider spawning over a Double Season in play is very often the decisive move that endorses the victory (only one time until now* I lost a game with Spider Spawming/Double Season combo againts a white/red deck with two damned Anger of gods played in a row).
Then the mistreated Howlgesit. thanks to Doubling season, can grow to 6/4 or 8/6, so it becomes very difficult to block for everyone and it can be decisive in many situation and it's nice as a target for the
hungry Bloodflow Connoisseur.
You're right, Gravedigger is probably underpowered for a 4-drop (bad stats), but it can rescue starting from turn 4 what is gone in the graveyard for the Necromancer's Assistant or Satyr Wayfinder abilities and however it is a body for the "sacrifice" purpose.
Thanks for the contribution.
*=I've played with this deck only 12 times so I have no enough of statistical data, for example I never faced a strong blue deck