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A friend of mine who's better than me at the game pointed out an idea that hadn't occurred to me, and encouraged me to try it since it'll be fairly cheap to build.  Basically, it's blackish aggro powered by Heartless Summoning and newly cheapened flying fatties.  I seek advice because the deck seems to call to me for a second color -- but I don't know which.  The skeleton looks like this:

4 Heartless Summoning
3 Bloodgift Demon
3 Desecration Demon
2 Nefarox
2 Griselbrand
2 Reaper from the Abyss
2 Necropolis Regent

And some utility cards like Murder and Duress .

As for the second color, my initial thought is to just go suicide black and do some early disruption with discard and creature destruction, and then lay the heavies and start punching them in the life total.

But if I go blue -- countermagic is always great, and then there's bounce.  If I go white, that's even more and better creature control against the field -- Pacifism is pretty rude vs. Thragtusk and makes sure Falkenrath Aristocrat gets only a single swing.  Also opens up Unburial Rites and Lingering Souls for chump-blocking.  If I go green, there's mana acceleration for if I don't pull Heartless, and I can play with cards like Sheltering Word to protect the demons.  Red gives great meta with Pillar of Flame , Slaughter Games , and Havoc Festival .

What do you think I should do for the rest of the deck?

While I don't necessarily agree that this deck is too slow (T2 Summoning, T3 Bloodgift 4/3 flyer?), it is definitely not ready for any second color unless it is splashed lightly--your creature costs are very demanding of black mana so it will be difficult to play consistently with anything else.

Duress is good.

Duty-Bound Dead is pretty terrible but his exalted trigger can be decent if you need low-drops urgently, for whatever reason. Regenerating blocker in a pinch. Most likely don't want to play this or any other 1-2 drop creatures.

Diregraf Ghoul and friends are meh. It (hopefully) turns into a 1/1 before you can attack with it. 

What you do need is more things to curve out from a T2 Summoning, to alleviate the problem of slowness:
Fiend of the Shadows may or may not be preferable to Shimian Specter .
Dread Slaver shuts down early ground aggression but isn't that great otherwise. 
If you choose to splash , Havengul Lich and Grimgrin, Corpse-born open up.

I like the idea of this deck. Heartless summoning seems so cool to me.
Have you thought about the following two cards.
Bitterheart Witch
Curse of Death's Hold

You might even want to try fitting in either 1 Curse of Misfortunes , 1 Curse of Echoes , or Curse of Exhaustion ...or all three, one of each.

I like the following setup. Let me know what you think. It's based largely on your concoction. 

4 x Heartless Summoning
3 x Curse of Death's Hold
1 x Curse of Misfortunes
1 x Curse of Exhaustion

4 x Bitterheart Witch

4 x Slitherhead
4 x Diregraf Ghoul

2 x Bloodgift Demon
2 x Desecration Demon
2 x Bloodline Keeper

4 x Murder
3 x Dead Weight
3 x Sign In Blood

4 x Isolated Chapel
3 x Cathedral of War
2 x Vault of the Archangel
14 x Swamp
 

I put in the 4 Slitherhead because it is useful in the graveyard if it dies on turn 2 AND it's a 1 mana +1 counter late game. Slitherhead and the Ghoul are just blockers in the beginning really. The Ghoul will survive the Summoning and can still block. 

7 Removal Spells. Murder....is murder and kills anything right? Dead Weight....just goes with the flavor of Death's Hold. Desecration Demon can overcome the counters...if they still have creatures. And he's a 6/6 for 4 even if you don't get the summoning so duh. The bloodgift demon isn't really as fast as the DDemon...but he can dig into your deck. Might want to fiddle around with him. Bloodline keeper gives you chump blockers and isn't TOO slow. You might get her out if you can stall with removal and the 1 drops.

I like that if you get the summoning out and you have a witch you can start cursing them. If you get the 1 misfortunes out it does the work of fetching the other curses but if you need help ASAP with creatures you can fetch Death's Hold. 

Well, let me know what you think. It was fun looking for cards for this deck. I really like your idea. Seems really fun to play. 

You have to have an alternate means of accomplishing your strategy; you can't rely on one play-set of cards. Unless of course your okay with losing more often than not.

Unburial Rites is one way, but it requires self-mill and that means . Of course, Rites itself begs for to be played.

The alternative is to build a deck that can compete without Heartless Summoning and use just a few fatties for when you do pull it. I don't know if that is really a vaible strategy.